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MySQL 8.4 Reference Manual / Functions and Operators / Cast Functions and Operators

14.10 Cast 函数和运算符

Table 14.15 Cast Functions and Operators

Name	Description	Deprecated
`BINARY`	Cast a string to a binary string	Yes
`CAST()`	Cast a value as a certain type
`CONVERT()`	Cast a value as a certain type

Cast functions and operators enable conversion of values from one data type to another.

BINARY expr

The BINARY operator converts the expression to a binary string (a string that has the binary character set and binary collation). A common use for BINARY is to force a character string comparison to be done byte by byte using numeric byte values rather than character by character. The BINARY operator also causes trailing spaces in comparisons to be significant. For information about the differences between the binary collation of the binary character set and the _bin collations of nonbinary character sets, see Section 12.8.5, “The binary Collation Compared to _bin Collations”.

The BINARY operator is deprecated; you should expect its removal in a future version of MySQL. Use CAST(... AS BINARY) instead.
```
mysql> SET NAMES utf8mb4 COLLATE utf8mb4_general_ci;
        -> OK
mysql> SELECT 'a' = 'A';
        -> 1
mysql> SELECT BINARY 'a' = 'A';
        -> 0
mysql> SELECT 'a' = 'a ';
        -> 1
mysql> SELECT BINARY 'a' = 'a ';
        -> 0
```
In a comparison, BINARY affects the entire operation; it can be given before either operand with the same result.

To convert a string expression to a binary string, these constructs are equivalent:
```
CONVERT(expr USING BINARY)
CAST(expr AS BINARY)
BINARY expr
```
If a value is a string literal, it can be designated as a binary string without converting it by using the _binary character set introducer:
```
mysql> SELECT 'a' = 'A';
        -> 1
mysql> SELECT _binary 'a' = 'A';
        -> 0
```
For information about introducers, see Section 12.3.8, “Character Set Introducers”.

The BINARY operator in expressions differs in effect from the BINARY attribute in character column definitions. For a character column defined with the BINARY attribute, MySQL assigns the table default character set and the binary (_bin) collation of that character set. Every nonbinary character set has a _bin collation. For example, if the table default character set is utf8mb4, these two column definitions are equivalent:
```
CHAR(10) BINARY
CHAR(10) CHARACTER SET utf8mb4 COLLATE utf8mb4_bin
```
The use of CHARACTER SET binary in the definition of a CHAR, VARCHAR, or TEXT column causes the column to be treated as the corresponding binary string data type. For example, the following pairs of definitions are equivalent:
```
CHAR(10) CHARACTER SET binary
BINARY(10)

VARCHAR(10) CHARACTER SET binary
VARBINARY(10)

TEXT CHARACTER SET binary
BLOB
```
If BINARY is invoked from within the mysql client, binary strings display using hexadecimal notation, depending on the value of the --binary-as-hex. For more information about that option, see Section 6.5.1, “mysql — The MySQL Command-Line Client”.
CAST(expr AS type [ARRAY])

CAST(timestamp_value AT TIME ZONE timezone_specifier AS DATETIME[(precision)])

timezone_specifier: [INTERVAL] '+00:00' | 'UTC'

With CAST(expr AS type syntax, the CAST() function takes an expression of any type and produces a result value of the specified type. This operation may also be expressed as CONVERT(expr, type), which is equivalent. If expr is NULL, CAST() returns NULL.

These type values are permitted:
- BINARY[(N)]
  
  Produces a string with the VARBINARY data type, except that when the expression expr is empty (zero length), the result type is BINARY(0). If the optional length N is given, BINARY(N) causes the cast to use no more than N bytes of the argument. Values shorter than N bytes are padded with 0x00 bytes to a length of N. If the optional length N is not given, MySQL calculates the maximum length from the expression. If the supplied or calculated length is greater than an internal threshold, the result type is BLOB. If the length is still too long, the result type is LONGBLOB.
  
  For a description of how casting to BINARY affects comparisons, see Section 13.3.3, “The BINARY and VARBINARY Types”.
- CHAR[(N)] [charset_info]
  
  Produces a string with the VARCHAR data type, unless the expression expr is empty (zero length), in which case the result type is CHAR(0). If the optional length N is given, CHAR(N) causes the cast to use no more than N characters of the argument. No padding occurs for values shorter than N characters. If the optional length N is not given, MySQL calculates the maximum length from the expression. If the supplied or calculated length is greater than an internal threshold, the result type is TEXT. If the length is still too long, the result type is LONGTEXT.
  
  With no charset_info clause, CHAR produces a string with the default character set. To specify the character set explicitly, these charset_info values are permitted:
  - CHARACTER SET charset_name: Produces a string with the given character set.
  - ASCII: Shorthand for CHARACTER SET latin1.
  - UNICODE: Shorthand for CHARACTER SET ucs2.
  In all cases, the string has the character set default collation.
- DATE
  
  Produces a DATE value.
- DATETIME[(M)]
  
  Produces a DATETIME value. If the optional M value is given, it specifies the fractional seconds precision.
- DECIMAL[(M[,D])]
  
  Produces a DECIMAL value. If the optional M and D values are given, they specify the maximum number of digits (the precision) and the number of digits following the decimal point (the scale). If D is omitted, 0 is assumed. If M is omitted, 10 is assumed.
- DOUBLE
  
  Produces a DOUBLE result.
- FLOAT[(p)]
  
  If the precision p is not specified, produces a result of type FLOAT. If p is provided and 0 <= < p <= 24, the result is of type FLOAT. If 25 <= p <= 53, the result is of type DOUBLE. If p < 0 or p > 53, an error is returned.
- JSON
  
  Produces a JSON value. For details on the rules for conversion of values between JSON and other types, see Comparison and Ordering of JSON Values.
- NCHAR[(N)]
  
  Like CHAR, but produces a string with the national character set. See Section 12.3.7, “The National Character Set”.
  
  Unlike CHAR, NCHAR does not permit trailing character set information to be specified.
- REAL
  
  Produces a result of type REAL. This is actually FLOAT if the REAL_AS_FLOAT SQL mode is enabled; otherwise the result is of type DOUBLE.
- SIGNED [INTEGER]
  
  Produces a signed BIGINT value.
- spatial_type
  
  CAST() and CONVERT() support casting geometry values from one spatial type to another, for certain combinations of spatial types. For details, see Cast Operations on Spatial Types.
- TIME[(M)]
  
  Produces a TIME value. If the optional M value is given, it specifies the fractional seconds precision.
- UNSIGNED [INTEGER]
  
  Produces an unsigned BIGINT value.
- YEAR
  
  Produces a YEAR value. These rules govern conversion to YEAR as follows:
  - For a four-digit number in the range 1901-2155 inclusive, or for a string which can be interpreted as a four-digit number in this range, return the corresponding YEAR value.
  - For a number consisting of one or two digits, or for a string which can be interpreted as such a number, return a YEAR value as follows:
    
    If the number is in the range 1-69 inclusive, add 2000 and return the sum.
    
    If the number is in the range 70-99 inclusive, add 1900 and return the sum.
  - For a string which evaluates to 0, return 2000.
  - For the number 0, return 0.
  - For a DATE, DATETIME, or TIMESTAMP value, return the YEAR portion of the value. For a TIME value, return the current year.
    
    If you do not specify the type of a TIME argument, you may get a different result from what you expect, as shown here:
    
    mysql> SELECT CAST("11:35:00" AS YEAR), CAST(TIME "11:35:00" AS YEAR); +--------------------------+-------------------------------+ | CAST("11:35:00" AS YEAR) | CAST(TIME "11:35:00" AS YEAR) | +--------------------------+-------------------------------+ | 2011 | 2021 | +--------------------------+-------------------------------+
  - If the argument is of type DECIMAL, DOUBLE, DECIMAL, or REAL, round the value to the nearest integer, then attempt to cast the value to YEAR using the rules for integer values, as shown here:
    
    mysql> SELECT CAST(1944.35 AS YEAR), CAST(1944.50 AS YEAR); +-----------------------+-----------------------+ | CAST(1944.35 AS YEAR) | CAST(1944.50 AS YEAR) | +-----------------------+-----------------------+ | 1944 | 1945 | +-----------------------+-----------------------+ mysql> SELECT CAST(66.35 AS YEAR), CAST(66.50 AS YEAR); +---------------------+---------------------+ | CAST(66.35 AS YEAR) | CAST(66.50 AS YEAR) | +---------------------+---------------------+ | 2066 | 2067 | +---------------------+---------------------+
  - An argument of type GEOMETRY cannot be converted to YEAR.
  - For a value that cannot be successfully converted to YEAR, return NULL.
  A string value containing non-numeric characters which must be truncated prior to conversion raises a warning, as shown here:
```
mysql> SELECT CAST("1979aaa" AS YEAR);
+-------------------------+
| CAST("1979aaa" AS YEAR) |
+-------------------------+
|                    1979 |
+-------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> SHOW WARNINGS;
+---------+------+-------------------------------------------+
| Level   | Code | Message                                   |
+---------+------+-------------------------------------------+
| Warning | 1292 | Truncated incorrect YEAR value: '1979aaa' |
+---------+------+-------------------------------------------+
```
InnoDB allows the use of an additional ARRAY keyword for creating a multi-valued index on a JSON array as part of CREATE INDEX, CREATE TABLE, and ALTER TABLE statements. ARRAY is not supported except when used to create a multi-valued index in one of these statements, in which case it is required. The column being indexed must be a column of type JSON. With ARRAY, the type following the AS keyword may specify any of the types supported by CAST(), with the exceptions of BINARY, JSON, and YEAR. For syntax information and examples, as well as other relevant information, see Multi-Valued Indexes.

Note

CONVERT(), unlike CAST(), does not support multi-valued index creation or the ARRAY keyword.

CAST() supports retrieval of a TIMESTAMP value as being in UTC, using the AT TIMEZONE operator. The only supported time zone is UTC; this can be specified as either of '+00:00' or 'UTC'. The only return type supported by this syntax is DATETIME, with an optional precision specifier in the range of 0 to 6, inclusive.

TIMESTAMP values that use timezone offsets are also supported.
```
mysql> SELECT @@system_time_zone;
+--------------------+
| @@system_time_zone |
+--------------------+
| EDT                |
+--------------------+
1 row in set (0.00 sec)

mysql> CREATE TABLE tz (c TIMESTAMP);
Query OK, 0 rows affected (0.41 sec)

mysql> INSERT INTO tz VALUES
    ->     ROW(CURRENT_TIMESTAMP),
    ->     ROW('2020-07-28 14:50:15+1:00');
Query OK, 1 row affected (0.08 sec)

mysql> TABLE tz;
+---------------------+
| c                   |
+---------------------+
| 2020-07-28 09:22:41 |
| 2020-07-28 09:50:15 |
+---------------------+
2 rows in set (0.00 sec)

mysql> SELECT CAST(c AT TIME ZONE '+00:00' AS DATETIME) AS u FROM tz;
+---------------------+
| u                   |
+---------------------+
| 2020-07-28 13:22:41 |
| 2020-07-28 13:50:15 |
+---------------------+
2 rows in set (0.00 sec)

mysql> SELECT CAST(c AT TIME ZONE 'UTC' AS DATETIME(2)) AS u FROM tz;
+------------------------+
| u                      |
+------------------------+
| 2020-07-28 13:22:41.00 |
| 2020-07-28 13:50:15.00 |
+------------------------+
2 rows in set (0.00 sec)
```
If you use 'UTC' as the time zone specifier with this form of CAST(), and the server raises an error such as Unknown or incorrect time zone: 'UTC', you may need to install the MySQL time zone tables (see Populating the Time Zone Tables).

AT TIME ZONE does not support the ARRAY keyword, and is not supported by the CONVERT() function.
CONVERT(expr USING transcoding_name)

CONVERT(expr,type)

CONVERT(expr USING transcoding_name) is standard SQL syntax. The non-USING form of CONVERT() is ODBC syntax. Regardless of the syntax used, the function returns NULL if expr is NULL.

CONVERT(expr USING transcoding_name) converts data between different character sets. In MySQL, transcoding names are the same as the corresponding character set names. For example, this statement converts the string 'abc' in the default character set to the corresponding string in the utf8mb4 character set:
```
SELECT CONVERT('abc' USING utf8mb4);
```
CONVERT(expr, type) syntax (without USING) takes an expression and a type value specifying a result type, and produces a result value of the specified type. This operation may also be expressed as CAST(expr AS type), which is equivalent. For more information, see the description of CAST().

CONVERT() with a USING clause converts data between character sets:

CONVERT(expr USING transcoding_name)

In MySQL, transcoding names are the same as the corresponding character set names.

Examples:

SELECT CONVERT('test' USING utf8mb4);
SELECT CONVERT(_latin1'Müller' USING utf8mb4);
INSERT INTO utf8mb4_table (utf8mb4_column)
    SELECT CONVERT(latin1_column USING utf8mb4) FROM latin1_table;

To convert strings between character sets, you can also use CONVERT(expr, type) syntax (without USING), or CAST(expr AS type), which is equivalent:

CONVERT(string, CHAR[(N)] CHARACTER SET charset_name)
CAST(string AS CHAR[(N)] CHARACTER SET charset_name)

Examples:

SELECT CONVERT('test', CHAR CHARACTER SET utf8mb4);
SELECT CAST('test' AS CHAR CHARACTER SET utf8mb4);

If you specify CHARACTER SET charset_name as just shown, the character set and collation of the result are charset_name and the default collation of charset_name. If you omit CHARACTER SET charset_name, the character set and collation of the result are defined by the character_set_connection and collation_connection system variables that determine the default connection character set and collation (see Section 12.4, “Connection Character Sets and Collations”).

A COLLATE clause is not permitted within a CONVERT() or CAST() call, but you can apply it to the function result. For example, these are legal:

SELECT CONVERT('test' USING utf8mb4) COLLATE utf8mb4_bin;
SELECT CONVERT('test', CHAR CHARACTER SET utf8mb4) COLLATE utf8mb4_bin;
SELECT CAST('test' AS CHAR CHARACTER SET utf8mb4) COLLATE utf8mb4_bin;

But these are illegal:

SELECT CONVERT('test' USING utf8mb4 COLLATE utf8mb4_bin);
SELECT CONVERT('test', CHAR CHARACTER SET utf8mb4 COLLATE utf8mb4_bin);
SELECT CAST('test' AS CHAR CHARACTER SET utf8mb4 COLLATE utf8mb4_bin);

For string literals, another way to specify the character set is to use a character set introducer. _latin1 and _latin2 in the preceding example are instances of introducers. Unlike conversion functions such as CAST(), or CONVERT(), which convert a string from one character set to another, an introducer designates a string literal as having a particular character set, with no conversion involved. For more information, see Section 12.3.8, “Character Set Introducers”.

Normally, you cannot compare a BLOB value or other binary string in case-insensitive fashion because binary strings use the binary character set, which has no collation with the concept of lettercase. To perform a case-insensitive comparison, first use the CONVERT() or CAST() function to convert the value to a nonbinary string. Comparisons of the resulting string use its collation. For example, if the conversion result collation is not case-sensitive, a LIKE operation is not case-sensitive. That is true for the following operation because the default utf8mb4 collation (utf8mb4_0900_ai_ci) is not case-sensitive:

SELECT 'A' LIKE CONVERT(blob_col USING utf8mb4)
  FROM tbl_name;

To specify a particular collation for the converted string, use a COLLATE clause following the CONVERT() call:

SELECT 'A' LIKE CONVERT(blob_col USING utf8mb4) COLLATE utf8mb4_unicode_ci
  FROM tbl_name;

To use a different character set, substitute its name for utf8mb4 in the preceding statements (and similarly to use a different collation).

CONVERT() and CAST() can be used more generally for comparing strings represented in different character sets. For example, a comparison of these strings results in an error because they have different character sets:

mysql> SET @s1 = _latin1 'abc', @s2 = _latin2 'abc';
mysql> SELECT @s1 = @s2;
ERROR 1267 (HY000): Illegal mix of collations (latin1_swedish_ci,IMPLICIT)
and (latin2_general_ci,IMPLICIT) for operation '='

Converting one of the strings to a character set compatible with the other enables the comparison to occur without error:

mysql> SELECT @s1 = CONVERT(@s2 USING latin1);
+---------------------------------+
| @s1 = CONVERT(@s2 USING latin1) |
+---------------------------------+
|                               1 |
+---------------------------------+

Character set conversion is also useful preceding lettercase conversion of binary strings. LOWER() and UPPER() are ineffective when applied directly to binary strings because the concept of lettercase does not apply. To perform lettercase conversion of a binary string, first convert it to a nonbinary string using a character set appropriate for the data stored in the string:

mysql> SET @str = BINARY 'New York';
mysql> SELECT LOWER(@str), LOWER(CONVERT(@str USING utf8mb4));
+-------------+------------------------------------+
| LOWER(@str) | LOWER(CONVERT(@str USING utf8mb4)) |
+-------------+------------------------------------+
| New York    | new york                           |
+-------------+------------------------------------+

Be aware that if you apply BINARY, CAST(), or CONVERT() to an indexed column, MySQL may not be able to use the index efficiently.

CAST() and CONVERT() support casting geometry values from one spatial type to another, for certain combinations of spatial types. The following list shows the permitted type combinations, where “MySQL extension” designates casts implemented in MySQL beyond those defined in the SQL/MM standard:

From Point to:
- MultiPoint
- GeometryCollection
From LineString to:
- Polygon (MySQL extension)
- MultiPoint (MySQL extension)
- MultiLineString
- GeometryCollection
From Polygon to:
- LineString (MySQL extension)
- MultiLineString (MySQL extension)
- MultiPolygon
- GeometryCollection
From MultiPoint to:
- Point
- LineString (MySQL extension)
- GeometryCollection
From MultiLineString to:
- LineString
- Polygon (MySQL extension)
- MultiPolygon (MySQL extension)
- GeometryCollection
From MultiPolygon to:
- Polygon
- MultiLineString (MySQL extension)
- GeometryCollection
From GeometryCollection to:
- Point
- LineString
- Polygon
- MultiPoint
- MultiLineString
- MultiPolygon

In spatial casts, GeometryCollection and GeomCollection are synonyms for the same result type.

Some conditions apply to all spatial type casts, and some conditions apply only when the cast result is to have a particular spatial type. For information about terms such as “well-formed geometry,” see Section 13.4.4, “Geometry Well-Formedness and Validity”.

These conditions apply to all spatial casts regardless of the result type:

The result of a cast is in the same SRS as that of the expression to cast.
Casting between spatial types does not change coordinate values or order.
If the expression to cast is NULL, the function result is NULL.
Casting to spatial types using the JSON_VALUE() function with a RETURNING clause specifying a spatial type is not permitted.
Casting to an ARRAY of spatial types is not permitted.
If the spatial type combination is permitted but the expression to cast is not a syntactically well-formed geometry, an ER_GIS_INVALID_DATA error occurs.
If the spatial type combination is permitted but the expression to cast is a syntactically well-formed geometry in an undefined spatial reference system (SRS), an ER_SRS_NOT_FOUND error occurs.
If the expression to cast has a geographic SRS but has a longitude or latitude that is out of range, an error occurs:
- If a longitude value is not in the range (−180, 180], an ER_GEOMETRY_PARAM_LONGITUDE_OUT_OF_RANGE error occurs.
- If a latitude value is not in the range [−90, 90], an ER_GEOMETRY_PARAM_LATITUDE_OUT_OF_RANGE error occurs.
Ranges shown are in degrees. If an SRS uses another unit, the range uses the corresponding values in its unit. The exact range limits deviate slightly due to floating-point arithmetic.

When the cast result type is Point, these conditions apply:

If the expression to cast is a well-formed geometry of type Point, the function result is that Point.
If the expression to cast is a well-formed geometry of type MultiPoint containing a single Point, the function result is that Point. If the expression contains more than one Point, an ER_INVALID_CAST_TO_GEOMETRY error occurs.
If the expression to cast is a well-formed geometry of type GeometryCollection containing only a single Point, the function result is that Point. If the expression is empty, contains more than one Point, or contains other geometry types, an ER_INVALID_CAST_TO_GEOMETRY error occurs.
If the expression to cast is a well-formed geometry of type other than Point, MultiPoint, GeometryCollection, an ER_INVALID_CAST_TO_GEOMETRY error occurs.

When the cast result type is LineString, these conditions apply:

If the expression to cast is a well-formed geometry of type LineString, the function result is that LineString.
If the expression to cast is a well-formed geometry of type Polygon that has no inner rings, the function result is a LineString containing the points of the outer ring in the same order. If the expression has inner rings, an ER_INVALID_CAST_TO_GEOMETRY error occurs.
If the expression to cast is a well-formed geometry of type MultiPoint containing at least two points, the function result is a LineString containing the points of the MultiPoint in the order they appear in the expression. If the expression contains only one Point, an ER_INVALID_CAST_TO_GEOMETRY error occurs.
If the expression to cast is a well-formed geometry of type MultiLineString containing a single LineString, the function result is that LineString. If the expression contains more than one LineString, an ER_INVALID_CAST_TO_GEOMETRY error occurs.
If the expression to cast is a well-formed geometry of type GeometryCollection, containing only a single LineString, the function result is that LineString. If the expression is empty, contains more than one LineString, or contains other geometry types, an ER_INVALID_CAST_TO_GEOMETRY error occurs.
If the expression to cast is a well-formed geometry of type other than LineString, Polygon, MultiPoint, MultiLineString, or GeometryCollection, an ER_INVALID_CAST_TO_GEOMETRY error occurs.

When the cast result type is Polygon, these conditions apply:

If the expression to cast is a well-formed geometry of type LineString that is a ring (that is, the start and end points are the same), the function result is a Polygon with an outer ring consisting of the points of the LineString in the same order. If the expression is not a ring, an ER_INVALID_CAST_TO_GEOMETRY error occurs. If the ring is not in the correct order (the exterior ring must be counter-clockwise), an ER_INVALID_CAST_POLYGON_RING_DIRECTION error occurs.
If the expression to cast is a well-formed geometry of type Polygon, the function result is that Polygon.
If the expression to cast is a well-formed geometry of type MultiLineString where all elements are rings, the function result is a Polygon with the first LineString as outer ring and any additional LineString values as inner rings. If any element of the expression is not a ring, an ER_INVALID_CAST_TO_GEOMETRY error occurs. If any ring is not in the correct order (the exterior ring must be counter-clockwise, interior rings must be clockwise), an ER_INVALID_CAST_POLYGON_RING_DIRECTION error occurs.
If the expression to cast is a well-formed geometry of type MultiPolygon containing a single Polygon, the function result is that Polygon. If the expression contains more than one Polygon, an ER_INVALID_CAST_TO_GEOMETRY error occurs.
If the expression to cast is a well-formed geometry of type GeometryCollection containing only a single Polygon, the function result is that Polygon. If the expression is empty, contains more than one Polygon, or contains other geometry types, an ER_INVALID_CAST_TO_GEOMETRY error occurs.
If the expression to cast is a well-formed geometry of type other than LineString, Polygon, MultiLineString, MultiPolygon, or GeometryCollection, an ER_INVALID_CAST_TO_GEOMETRY error occurs.

When the cast result type is MultiPoint, these conditions apply:

If the expression to cast is a well-formed geometry of type Point, the function result is a MultiPoint containing that Point as its sole element.
If the expression to cast is a well-formed geometry of type LineString, the function result is a MultiPoint containing the points of the LineString in the same order.
If the expression to cast is a well-formed geometry of type MultiPoint, the function result is that MultiPoint.
If the expression to cast is a well-formed geometry of type GeometryCollection containing only points, the function result is a MultiPoint containing those points. If the GeometryCollection is empty or contains other geometry types, an ER_INVALID_CAST_TO_GEOMETRY error occurs.
If the expression to cast is a well-formed geometry of type other than Point, LineString, MultiPoint, or GeometryCollection, an ER_INVALID_CAST_TO_GEOMETRY error occurs.

When the cast result type is MultiLineString, these conditions apply:

If the expression to cast is a well-formed geometry of type LineString, the function result is a MultiLineString containing that LineString as its sole element.
If the expression to cast is a well-formed geometry of type Polygon, the function result is a MultiLineString containing the outer ring of the Polygon as its first element and any inner rings as additional elements in the order they appear in the expression.
If the expression to cast is a well-formed geometry of type MultiLineString, the function result is that MultiLineString.
If the expression to cast is a well-formed geometry of type MultiPolygon containing only polygons without inner rings, the function result is a MultiLineString containing the polygon rings in the order they appear in the expression. If the expression contains any polygons with inner rings, an ER_WRONG_PARAMETERS_TO_STORED_FCT error occurs.
If the expression to cast is a well-formed geometry of type GeometryCollection containing only linestrings, the function result is a MultiLineString containing those linestrings. If the expression is empty or contains other geometry types, an ER_INVALID_CAST_TO_GEOMETRY error occurs.
If the expression to cast is a well-formed geometry of type other than LineString, Polygon, MultiLineString, MultiPolygon, or GeometryCollection, an ER_INVALID_CAST_TO_GEOMETRY error occurs.

When the cast result type is MultiPolygon, these conditions apply:

If the expression to cast is a well-formed geometry of type Polygon, the function result is a MultiPolygon containing the Polygon as its sole element.
If the expression to cast is a well-formed geometry of type MultiLineString where all elements are rings, the function result is a MultiPolygon containing a Polygon with only an outer ring for each element of the expression. If any element is not a ring, an ER_INVALID_CAST_TO_GEOMETRY error occurs. If any ring is not in the correct order (exterior ring must be counter-clockwise), an ER_INVALID_CAST_POLYGON_RING_DIRECTION error occurs.
If the expression to cast is a well-formed geometry of type MultiPolygon, the function result is that MultiPolygon.
If the expression to cast is a well-formed geometry of type GeometryCollection containing only polygons, the function result is a MultiPolygon containing those polygons. If the expression is empty or contains other geometry types, an ER_INVALID_CAST_TO_GEOMETRY error occurs.
If the expression to cast is a well-formed geometry of type other than Polygon, MultiLineString, MultiPolygon, or GeometryCollection, an ER_INVALID_CAST_TO_GEOMETRY error occurs.

When the cast result type is GeometryCollection, these conditions apply:

GeometryCollection and GeomCollection are synonyms for the same result type.
If the expression to cast is a well-formed geometry of type Point, the function result is a GeometryCollection containing that Point as its sole element.
If the expression to cast is a well-formed geometry of type LineString, the function result is a GeometryCollection containing that LineString as its sole element.
If the expression to cast is a well-formed geometry of type Polygon, the function result is a GeometryCollection containing that Polygon as its sole element.
If the expression to cast is a well-formed geometry of type MultiPoint, the function result is a GeometryCollection containing the points in the order they appear in the expression.
If the expression to cast is a well-formed geometry of type MultiLineString, the function result is a GeometryCollection containing the linestrings in the order they appear in the expression.
If the expression to cast is a well-formed geometry of type MultiPolygon, the function result is a GeometryCollection containing the elements of the MultiPolygon in the order they appear in the expression.
If the expression to cast is a well-formed geometry of type GeometryCollection, the function result is that GeometryCollection.

The cast functions are useful for creating a column with a specific type in a CREATE TABLE ... SELECT statement:

mysql> CREATE TABLE new_table SELECT CAST('2000-01-01' AS DATE) AS c1;
mysql> SHOW CREATE TABLE new_table\G
*************************** 1. row ***************************
       Table: new_table
Create Table: CREATE TABLE `new_table` (
  `c1` date DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci

The cast functions are useful for sorting ENUM columns in lexical order. Normally, sorting of ENUM columns occurs using the internal numeric values. Casting the values to CHAR results in a lexical sort:

SELECT enum_col FROM tbl_name 
  ORDER BY CAST(enum_col AS CHAR);

CAST() also changes the result if you use it as part of a more complex expression such as CONCAT('Date: ',CAST(NOW() AS DATE)).

For temporal values, there is little need to use CAST() to extract data in different formats. Instead, use a function such as EXTRACT(), DATE_FORMAT(), or TIME_FORMAT(). See Section 14.7, “Date and Time Functions”.

To cast a string to a number, it normally suffices to use the string value in numeric context:

mysql> SELECT 1+'1';
       -> 2

That is also true for hexadecimal and bit literals, which are binary strings by default:

mysql> SELECT X'41', X'41'+0;
        -> 'A', 65
mysql> SELECT b'1100001', b'1100001'+0;
        -> 'a', 97

A string used in an arithmetic operation is converted to a floating-point number during expression evaluation.

A number used in string context is converted to a string:

mysql> SELECT CONCAT('hello you ',2);
        -> 'hello you 2'

For information about implicit conversion of numbers to strings, see Section 14.3, “Type Conversion in Expression Evaluation”.

MySQL supports arithmetic with both signed and unsigned 64-bit values. For numeric operators (such as + or -) where one of the operands is an unsigned integer, the result is unsigned by default (see Section 14.6.1, “Arithmetic Operators”). To override this, use the SIGNED or UNSIGNED cast operator to cast a value to a signed or unsigned 64-bit integer, respectively.

mysql> SELECT 1 - 2;
        -> -1
mysql> SELECT CAST(1 - 2 AS UNSIGNED);
        -> 18446744073709551615
mysql> SELECT CAST(CAST(1 - 2 AS UNSIGNED) AS SIGNED);
        -> -1

If either operand is a floating-point value, the result is a floating-point value and is not affected by the preceding rule. (In this context, DECIMAL column values are regarded as floating-point values.)

mysql> SELECT CAST(1 AS UNSIGNED) - 2.0;
        -> -1.0

The SQL mode affects the result of conversion operations (see Section 7.1.11, “Server SQL Modes”). Examples:

For conversion of a “zero” date string to a date, CONVERT() and CAST() return NULL and produce a warning when the NO_ZERO_DATE SQL mode is enabled.
For integer subtraction, if the NO_UNSIGNED_SUBTRACTION SQL mode is enabled, the subtraction result is signed even if any operand is unsigned.

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