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sql-statement-modify-column.md

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title summary aliases
MODIFY COLUMN
TiDB 数据库中 MODIFY COLUMN 的使用概况。
/docs-cn/dev/sql-statements/sql-statement-modify-column/
/docs-cn/dev/reference/sql/statements/modify-column/

MODIFY COLUMN

ALTER TABLE .. MODIFY COLUMN 语句用于修改已有表上的列，包括列的数据类型和属性。若要同时重命名，可改用 CHANGE COLUMN 语句。

语法图

AlterTableStmt ::=
    'ALTER' IgnoreOptional 'TABLE' TableName ( AlterTableSpecListOpt AlterTablePartitionOpt | 'ANALYZE' 'PARTITION' PartitionNameList ( 'INDEX' IndexNameList )? AnalyzeOptionListOpt )

AlterTableSpec ::=
    TableOptionList
|   'SET' 'TIFLASH' 'REPLICA' LengthNum LocationLabelList
|   'CONVERT' 'TO' CharsetKw ( CharsetName | 'DEFAULT' ) OptCollate
|   'ADD' ( ColumnKeywordOpt IfNotExists ( ColumnDef ColumnPosition | '(' TableElementList ')' ) | Constraint | 'PARTITION' IfNotExists NoWriteToBinLogAliasOpt ( PartitionDefinitionListOpt | 'PARTITIONS' NUM ) )
|   ( ( 'CHECK' | 'TRUNCATE' ) 'PARTITION' | ( 'OPTIMIZE' | 'REPAIR' | 'REBUILD' ) 'PARTITION' NoWriteToBinLogAliasOpt ) AllOrPartitionNameList
|   'COALESCE' 'PARTITION' NoWriteToBinLogAliasOpt NUM
|   'DROP' ( ColumnKeywordOpt IfExists ColumnName RestrictOrCascadeOpt | 'PRIMARY' 'KEY' | 'PARTITION' IfExists PartitionNameList | ( KeyOrIndex IfExists | 'CHECK' ) Identifier | 'FOREIGN' 'KEY' IfExists Symbol )
|   'EXCHANGE' 'PARTITION' Identifier 'WITH' 'TABLE' TableName WithValidationOpt
|   ( 'IMPORT' | 'DISCARD' ) ( 'PARTITION' AllOrPartitionNameList )? 'TABLESPACE'
|   'REORGANIZE' 'PARTITION' NoWriteToBinLogAliasOpt ReorganizePartitionRuleOpt
|   'ORDER' 'BY' AlterOrderItem ( ',' AlterOrderItem )*
|   ( 'DISABLE' | 'ENABLE' ) 'KEYS'
|   ( 'MODIFY' ColumnKeywordOpt IfExists | 'CHANGE' ColumnKeywordOpt IfExists ColumnName ) ColumnDef ColumnPosition
|   'ALTER' ( ColumnKeywordOpt ColumnName ( 'SET' 'DEFAULT' ( SignedLiteral | '(' Expression ')' ) | 'DROP' 'DEFAULT' ) | 'CHECK' Identifier EnforcedOrNot | 'INDEX' Identifier IndexInvisible )
|   'RENAME' ( ( 'COLUMN' | KeyOrIndex ) Identifier 'TO' Identifier | ( 'TO' | '='? | 'AS' ) TableName )
|   LockClause
|   AlgorithmClause
|   'FORCE'
|   ( 'WITH' | 'WITHOUT' ) 'VALIDATION'
|   'SECONDARY_LOAD'
|   'SECONDARY_UNLOAD'

ColumnKeywordOpt ::= 'COLUMN'?

ColumnDef ::=
    ColumnName ( Type | 'SERIAL' ) ColumnOptionListOpt

ColumnPosition ::=
    ( 'FIRST' | 'AFTER' ColumnName )?

示例

{{< copyable "sql" >}}

CREATE TABLE t1 (id int not null primary key AUTO_INCREMENT, col1 INT);
Query OK, 0 rows affected (0.11 sec)

{{< copyable "sql" >}}

INSERT INTO t1 (col1) VALUES (1),(2),(3),(4),(5);
Query OK, 5 rows affected (0.02 sec)
Records: 5  Duplicates: 0  Warnings: 0

{{< copyable "sql" >}}

ALTER TABLE t1 MODIFY col1 BIGINT;
Query OK, 0 rows affected (0.09 sec)

{{< copyable "sql" >}}

SHOW CREATE TABLE t1\G;
*************************** 1. row ***************************
       Table: t1
Create Table: CREATE TABLE `t1` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `col1` bigint(20) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin AUTO_INCREMENT=30001
1 row in set (0.00 sec)

MySQL 兼容性

  • 不支持在单个 ALTER TABLE 语句修改多个列，例如：

    ALTER TABLE t1 MODIFY col1 BIGINT, MODIFY id BIGINT NOT NULL;
ERROR 1105 (HY000): Unsupported multi schema change

  • 不支持有损变更，以及部分数据类型的更改（包括整数改为字符串或 BLOB 格式等）。例如：

    CREATE TABLE t1 (col1 BIGINT);
ALTER TABLE t1 MODIFY col1 INT;
ERROR 8200 (HY000): Unsupported modify column length 11 is less than origin 20

  • 不支持修改 decimal 类型的精度。例如：

    CREATE TABLE t (a DECIMAL(5, 3));
ALTER TABLE t MODIFY COLUMN a DECIMAL(6, 3);
ERROR 8200 (HY000): Unsupported modify column: can't change decimal column precision

另请参阅