diff --git a/functions-and-operators/cast-functions-and-operators.md b/functions-and-operators/cast-functions-and-operators.md index f012f2d8e2ba..cc8c9cd4d316 100644 --- a/functions-and-operators/cast-functions-and-operators.md +++ b/functions-and-operators/cast-functions-and-operators.md @@ -18,3 +18,13 @@ Cast 函数和操作符用于将某种数据类型的值转换为另一种数据 > **注意：** > > TiDB 和 MySQL 对于 `SELECT CAST(MeN AS CHAR)`（或者等价的 `SELECT CONVERT(MeM, CHAR)`）的结果显示不一致，其中 `MeN` 是用科学计数法表示的双精度浮点数。MySQL 在 `-15 <= N <= 14` 时显示完整数值，在 `N < -15` 或 `N > 14` 时显示科学计数法。而 TiDB 始终显示完整数值。例如，MySQL 对于 `SELECT CAST(3.1415e15 AS CHAR)` 的显示结果为 `3.1415e15`，而 TiDB 的显示结果为 `3141500000000000`。 + +## MySQL 兼容性 + +TiDB 不支持部分数据类型的变更，例如部分时间类型、Bit、Set、Enum 和 JSON 等。 + +```sql +CREATE TABLE t (a DECIMAL(13, 7)); +ALTER TABLE t CHANGE COLUMN a a DATETIME; +ERROR 8200 (HY000): Unsupported modify column: [ddl:8200]Unsupported modify column: change from original type decimal(13,7) to datetime is currently unsupported yet +``` \ No newline at end of file diff --git a/sql-statements/sql-statement-change-column.md b/sql-statements/sql-statement-change-column.md index a1950feeb093..be6f53525376 100644 --- a/sql-statements/sql-statement-change-column.md +++ b/sql-statements/sql-statement-change-column.md @@ -6,11 +6,11 @@ aliases: ['/docs-cn/dev/sql-statements/sql-statement-change-column/','/docs-cn/d # CHANGE COLUMN -`ALTER TABLE.. CHANGE COLUMN` 语句用于在已有表上更改列，包括对列进行重命名，和将数据改为兼容类型。 +`ALTER TABLE .. CHANGE COLUMN` 语句用于在已有表上更改列，包括对列进行重命名，和将数据改为兼容类型。 从 v5.1.0 版本起，TiDB 开始支持 Reorg 数据的类型变更，包括但不限于： -- 从 varchar 转换为 bigint +- 从 varchar 转换为 bigint - decimal 精度修改 - 从 varchar(10) 到 varchar(5) 的长度压缩 @@ -52,108 +52,137 @@ ColumnName ::= ## 示例 -{{< copyable "sql" >}} +创建表 `t1` 并插入数据： ```sql CREATE TABLE t1 (id int not null primary key AUTO_INCREMENT, col1 INT); +INSERT INTO t1 (col1) VALUES (1),(2),(3),(4),(5); +SELECT * FROM t1; ``` -``` -Query OK, 0 rows affected (0.11 sec) -``` - -{{< copyable "sql" >}} +表 `t1` 的数据如下： ```sql -INSERT INTO t1 (col1) VALUES (1),(2),(3),(4),(5); ++----+------+ +| id | col1 | ++----+------+ +| 1 | 1 | +| 2 | 2 | +| 3 | 3 | +| 4 | 4 | +| 5 | 5 | ++----+------+ +5 rows in set (0.01 sec) ``` -``` -Query OK, 5 rows affected (0.02 sec) -Records: 5 Duplicates: 0 Warnings: 0 -``` - -{{< copyable "sql" >}} +将表 `t1` 的列 `col1` 重命名为 `col2`： ```sql ALTER TABLE t1 CHANGE col1 col2 INT; +SELECT * FROM t1; ``` -``` -Query OK, 0 rows affected (0.09 sec) -``` - -{{< copyable "sql" >}} +表 `t1` 的数据如下： ```sql -ALTER TABLE t1 CHANGE col2 col3 BIGINT, ALGORITHM=INSTANT; ++----+------+ +| id | col2 | ++----+------+ +| 1 | 1 | +| 2 | 2 | +| 3 | 3 | +| 4 | 4 | +| 5 | 5 | ++----+------+ +5 rows in set (0.01 sec) ``` -``` -Query OK, 0 rows affected (0.08 sec) -``` - -{{< copyable "sql" >}} +将表 `t1` 的列 `col2` 重命名为 `col3`，并将列的类型从 `INT` 改为 `BIGINT`： ```sql -ALTER TABLE t1 CHANGE col3 col4 BIGINT, CHANGE id id2 INT NOT NULL; -``` - -``` -ERROR 1105 (HY000): can't run multi schema change +ALTER TABLE t1 CHANGE col2 col3 BIGINT, ALGORITHM=INSTANT; +SELECT * FROM t1; ``` -{{< copyable "sql" >}} +表 `t1` 的数据如下： ```sql -CREATE TABLE t (a int primary key); -ALTER TABLE t CHANGE COLUMN a a VARCHAR(10); ++----+------+ +| id | col3 | ++----+------+ +| 1 | 1 | +| 2 | 2 | +| 3 | 3 | +| 4 | 4 | +| 5 | 5 | ++----+------+ +5 rows in set (0.00 sec) ``` -``` -ERROR 8200 (HY000): Unsupported modify column: column has primary key flag -``` - -{{< copyable "sql" >}} +下面语句将表 `t1` 的列 `col3` 重命名为 `col4`，并将主键 `id` 重命名为 `id2`： ```sql -CREATE TABLE t (c1 INT, c2 INT, c3 INT) partition by range columns(c1) ( partition p0 values less than (10), partition p1 values less than (maxvalue)); -ALTER TABLE t CHANGE COLUMN c1 c1 DATETIME; -``` - -``` -ERROR 8200 (HY000): Unsupported modify column: table is partition table +ALTER TABLE t1 CHANGE col3 col4 BIGINT, CHANGE id id2 INT NOT NULL; +ERROR 8200 (HY000): Unsupported modify column: can't remove auto_increment without @@tidb_allow_remove_auto_inc enabled ``` -{{< copyable "sql" >}} +由于 `id` 是自增列，所以不能直接将其重命名为 `id2`。需要先开启 [`tidb_allow_remove_auto_inc`](/system-variables.md#tidb_allow_remove_auto_inc-从-v2118-和-v304-版本开始引入)，然后再执行 `ALTER TABLE` 语句： ```sql -CREATE TABLE t (a INT, b INT as (a+1)); -ALTER TABLE t CHANGE COLUMN b b VARCHAR(10); -``` - -``` -ERROR 8200 (HY000): Unsupported modify column: column is generated +SET tidb_allow_remove_auto_inc = ON; +ALTER TABLE t1 CHANGE col3 col4 BIGINT, CHANGE id id2 INT NOT NULL; +SELECT * FROM t1; ``` -{{< copyable "sql" >}} - -```sql -CREATE TABLE t (a DECIMAL(13, 7)); -ALTER TABLE t CHANGE COLUMN a a DATETIME; -``` +表 `t1` 的数据如下： ``` -ERROR 8200 (HY000): Unsupported modify column: change from original type decimal(13,7) to datetime is currently unsupported yet ++-----+------+ +| id2 | col4 | ++-----+------+ +| 1 | 1 | +| 2 | 2 | +| 3 | 3 | +| 4 | 4 | +| 5 | 5 | ++-----+------+ +5 rows in set (0.00 sec) ``` ## MySQL 兼容性 * 不支持主键列上 [Reorg-Data](/sql-statements/sql-statement-modify-column.md#reorg-data-change) 类型的变更。 + + ```sql + CREATE TABLE t (a int primary key); + ALTER TABLE t CHANGE COLUMN a a VARCHAR(10); + ERROR 8200 (HY000): Unsupported modify column: this column has primary key flag + ``` + * 不支持分区表上的列类型变更。 + + ```sql + CREATE TABLE t (c1 INT, c2 INT, c3 INT) partition by range columns(c1) ( partition p0 values less than (10), partition p1 values less than (maxvalue)); + ALTER TABLE t CHANGE COLUMN c1 c1 DATETIME; + ERROR 8200 (HY000): Unsupported modify column: table is partition table + ``` + * 不支持生成列上的列类型变更。 + + ```sql + CREATE TABLE t (a INT, b INT as (a+1)); + ALTER TABLE t CHANGE COLUMN b b VARCHAR(10); + ERROR 8200 (HY000): Unsupported modify column: newCol IsGenerated false, oldCol IsGenerated true + ``` + * 不支持部分数据类型（例如，部分时间类型、Bit、Set、Enum、JSON 等）的变更，因为 TiDB 中 `CAST` 函数与 MySQL 的行为存在兼容性问题。 + ```sql + CREATE TABLE t (a DECIMAL(13, 7)); + ALTER TABLE t CHANGE COLUMN a a DATETIME; + ERROR 8200 (HY000): Unsupported modify column: [ddl:8200]Unsupported modify column: change from original type decimal(13,7) to datetime is currently unsupported yet + ``` + ## 另请参阅 * [CREATE TABLE](/sql-statements/sql-statement-create-table.md)