簡述
MySQL支持update t1,t2 set t1.a=2;這種語法，別的關系數據庫例如oracle和sql server都不支持。這種語法有時候寫起來挺方便，但他有一個坑。
測試腳本
drop database fander;
create database fander;
use fander;
create table t1(a int);
create table t2(a int);
insert into t1 value(1);
select * from t1;
update t1,t2 set t1.a=2;
select * from t1;
測試結果
mysql> drop database fander;
create table t2(a int);
insert into t1 value(1);
select * from t1;
update t1,t2 set t1.a=2;
select * from t1;Query OK, 6 rows affected (0.35 sec)
mysql> create database fander;
Query OK, 1 row affected (0.00 sec)
mysql> use fander;
Database changed
mysql> create table t1(a int);
Query OK, 0 rows affected (0.11 sec)
mysql> create table t2(a int);
Query OK, 0 rows affected (0.23 sec)
mysql> insert into t1 value(1);
Query OK, 1 row affected (0.00 sec)
mysql> select * from t1;
+------+
| a |
+------+
| 1 |
+------+
1 row in set (0.00 sec)
mysql> update t1,t2 set t1.a=2;
Query OK, 0 rows affected (0.00 sec)
Rows matched: 0 Changed: 0 Warnings: 0
mysql> select * from t1;
+------+
| a |
+------+
| 1 |
+------+
1 row in set (0.00 sec)
mysql>
結果非預期
我們想更新t1表的a列值為2，但發現並沒有更新成功。。。
原因
因為t2是空表！
update t1,t2 set t1.a=2; 這個語句看起來修改與t2無關。t2是否空表應該不影響update行為才對呀？
https://dev.mysql.com/doc/refman/8.0/en/update.html
For the multiple-table syntax, UPDATE updates rows in each table named in table_references that satisfy the conditions. Each matching row is updated once, even if it matches the conditions multiple times. For multiple-table syntax, ORDER BY and LIMIT cannot be used.
官方手冊其實寫明了，update語法是根據table_references，即關聯后的表，再做更新的。
mysql> select * from t1;
+------+
| a |
+------+
| 1 |
+------+
1 row in set (0.00 sec)
mysql> select * from t1,t2;
Empty set (0.00 sec)
因為t2為空表，t1的數據和空表數據做笛卡爾積，關聯表后則為空。
update對這個空的table_references做更新，當然就找不到記錄，就沒有更新成功咯。
總結
這是一個坑，一般使用遇不到，但需要知道。