UPDATE
UPDATE 语句用于修改指定表中的数据。
语法图
- UpdateStmt
- UpdateOption
- TableRef
- TableRefs
UpdateStmt ::=
"UPDATE" UpdateOption
( TableRef "SET" Assignment ("," Assignment)* WhereClause? OrderBy? Limit?
| TableRefs "SET" Assignment ("," Assignment)* WhereClause?
)
UpdateOption ::=
OptimizerHints? ("LOW_PRIORITY" | "HIGH_PRIORITY" | "DELAYED")? "IGNORE"?
TableRef ::=
( TableFactor | JoinTable )
TableRefs ::=
EscapedTableRef ("," EscapedTableRef)*
示例
CREATE TABLE t1 (id INT NOT NULL PRIMARY KEY AUTO_INCREMENT, c1 INT NOT NULL);
Query OK, 0 rows affected (0.11 sec)
INSERT INTO t1 (c1) VALUES (1), (2), (3);
Query OK, 3 rows affected (0.02 sec)
Records: 3 Duplicates: 0 Warnings: 0
SELECT * FROM t1;
+----+----+
| id | c1 |
+----+----+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+----+----+
3 rows in set (0.00 sec)
UPDATE t1 SET c1=5 WHERE c1=3;
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0
SELECT * FROM t1;
+----+----+
| id | c1 |
+----+----+
| 1 | 1 |
| 2 | 2 |
| 3 | 5 |
+----+----+
3 rows in set (0.00 sec)
MySQL 兼容性
在计算表达式中的列时，TiDB 总使用原始的值。例如：
CREATE TABLE t (a int, b int);
INSERT INTO t VALUES (1,2);
UPDATE t SET a = a+1,b=a;
在 MySQL 中，b 列的值会被更新成 2，因为 b 列被设置为与 a 列相同，而 a（最初是 1）在同一条语句中被更新成了 2。
TiDB 遵守标准的 SQL 行为，这里将 b 列值更新成 1。