mysql - SQL 语句 : how can I pick a value only once from a table to update another table?

Question

我希望用匹配相同代码的 table2 的任何不同值更新 table1，无论是哪一个或顺序(来自 table2 的值不能被选择多次)

+-------------------+      +--------------+
|      table1       |      |    table2    |
+-------------------+      +--------------+
| id | code | value |      | code | value |
+----+------+-------+      +------+-------+
| 1  |  A   |       | <--  |  A   |  v1   |
| 2  |  A   |       |      |  B   |  v2   |
| 3  |  B   |       |      |  A   |  v3   |
+----+------+-------+      |  A   |  v5   |
                           |  A   |  v6   |
                           +------+-------+
+-------------------+                      
|      table1       |                      
+-------------------+                      
| id | code | value |
+----+------+-------+
| 1  |  A   |  v6   |
| 2  |  A   |  v3   |
| 3  |  B   |  v2   |
+----+------+-------+

SQL更新语句怎么写？ (MySQL 5.7)

Answer 1

这需要 Row_Number()窗口函数的魔力!不幸的是，您的 MySQL 版本是 5.7；所以使用 User-defined variables 的更详细的解决方案如下:

UPDATE 
table1 AS t1 

JOIN 

(SELECT 
   dt1.id, 
   IF(@cd1 = dt1.code, @rn1 := @rn1 + 1, 1) AS row_num1, 
   @cd1 := dt1.code AS code 
 FROM (SELECT id, code FROM table1 ORDER BY code, id) AS dt1
 CROSS JOIN (SELECT @rn1 := 0, @cd1 := '') AS init1
) AS t2 
  ON t2.id = t1.id 

JOIN 

(SELECT 
   IF(@cd2 = dt2.code, @rn2 := @rn2 + 1, 1) AS row_num2, 
   @cd2 := dt2.code AS code, 
   dt2.value 
 FROM (SELECT code, value FROM table2 ORDER BY code) AS dt2
 CROSS JOIN (SELECT @rn2 := 0, @cd2 := '') AS init2
) AS t3 
  ON t3.row_num2 = t2.row_num1 AND 
     t3.code = t2.code 

SET t1.value = t3.value 

你可以在这个 answer 中查看类似技术的解释。 .