MySQL（MyISAM） - 将字段更新为来自不同表的两个字段中的最大字段

Question

我有两个表，t1和t2，每个表有两列 - id_user和age。
如果在t2中没有匹配的ID，如何将t1.age更新为最大的t1.age和t2.age以匹配ID并保持t1.age不变。

更新前：

 
t1 
+-------+---+ 
|id_user|age| 
+-------+---+ 
|  1| 5| 
+-------+---+ 
|  2| 10| 
+-------+---+ 
|  3| 10| 
+-------+---+ 

t2 
+-------+---+ 
|id_user|age| 
+-------+---+ 
|  2| 12| 
+-------+---+ 
|  3| 8| 
+-------+---+ 
|  4| 20| 
+-------+---+ 

更新后：

 
t1 
+-------+---+ 
|id_user|age| 
+-------+---+ 
|  1| 5| 
+-------+---+ 
|  2| 12| 
+-------+---+ 
|  3| 10| 
+-------+---+ 

Answer 1

您可能也想尝试：

UPDATE t1 
JOIN t2 ON (t2.id_user = t1.id_user) 
SET  t1.age = t2.age 
WHERE t2.age > t1.age; 

测试用例：

CREATE TABLE t1 (id_user int, age int); 
CREATE TABLE t2 (id_user int, age int); 

INSERT INTO t1 VALUES (1, 5); 
INSERT INTO t1 VALUES (2, 10); 
INSERT INTO t1 VALUES (3, 10); 

INSERT INTO t2 VALUES (2, 12); 
INSERT INTO t2 VALUES (3, 8); 
INSERT INTO t2 VALUES (4, 20); 

结果：

SELECT * FROM t1; 
+---------+------+ 
| id_user | age | 
+---------+------+ 
|  1 | 5 | 
|  2 | 12 | 
|  3 | 10 | 
+---------+------+ 
3 rows in set (0.00 sec) 

Answer 2

UPDATE t1 
SET age = T2.age 
FROM t1 
INNER JOIN t2 
ON t2.id_user = t1.id_user 
WHERE t2.age > t1.age