再論關聯子查詢更新

之前使用oracle，遇到過2個表進行關聯子查詢更新的坑，沒有加exists從句，導致了不匹配的行設定成null,這個問題在mysql中也是一樣的。下面是測試

mysql> select * from test1;

+------+------+
| id | name |
+------+------+
| 1 | dddd |
| 2 | null |
| 3 | null |
+------+------+
3 rows in set (0.00 sec)
mysql> select * from test2;
+------+-------+
| id | name2 |
+------+-------+
| 1 | dddd |
+------+-------+
1 row in set (0.00 sec)
mysql> update test1 set name=(select name2 from test2 where test1.id=test2.id);
query ok, 3 rows affected (0.00 sec)
rows matched: 3 changed: 3 warnings: 0
mysql> select * from test1;
+------+--------+
| id | name |
+------+--------+
| 1 | ffffff |
| 2 | null |
| 3 | null |
+------+--------+
重置資料後
mysql> update test1 set name=(select name2 from test2 where test1.id=test2.id) where exists(select 1 from test2 where test1.id=test2.id);
query ok, 1 row affected (0.00 sec)
rows matched: 1 changed: 1 warnings: 0
mysql> select * from tes1;
error 1146 (42s02): table 'test.tes1' doesn't exist
mysql> select * from test1;
+------+------+
| id | name |
+------+------+
| 1 | ffff |
| 2 | bb |
| 3 | cc |
+------+------+
3 rows in set (0.01 sec)

mysql> insert into test1 values(1,'aa');

query ok, 1 row affected (0.00 sec)
mysql> insert into test1 values(2,'bb');
query ok, 1 row affected (0.00 sec)
mysql> insert into test1 values(3,'cc');
query ok, 1 row affected (0.00 sec)
mysql> select * from test2;
empty set (0.00 sec)
mysql> insert into test2 values(1,'eeee');
query ok, 1 row affected (0.00 sec)
mysql> insert into test2 values(2,'ffffff');
query ok, 1 row affected (0.00 sec)
mysql> insert into test2 values(3,'cc');
query ok, 1 row affected (0.01 sec)
mysql> update test1 set name=(select name2 from test2 where test1.id=test2.id);
query ok, 2 rows affected (0.01 sec)
rows matched: 3 changed: 2 warnings: 0
mysql> select * from test1;
+------+--------+
| id | name |
+------+--------+
| 1 | eeee |
| 2 | ffffff |
| 3 | cc |
+------+--------+

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