MySQL学习笔记：从一个表update到另外一个表

# ---- 测试数据 ----
# 表1
CREATE TABLE temp_x AS
    SELECT 1 AS c_id, 1.11 AS c_amount FROM DUAL
UNION ALL
    SELECT 2 AS c_id, 1.22 AS c_amount FROM DUAL;
# 表2
CREATE TABLE temp_y AS
    SELECT 1 AS c_id, 1.43 AS c_amount FROM DUAL
UNION ALL
    SELECT 2 AS c_id, 1.44 AS c_amount FROM DUAL;

# 查询
SELECT * FROM temp_x;
SELECT * FROM temp_y;

# 恢复数据
UPDATE temp_x SET c_amount = 1.11 WHERE c_id = 1;
UPDATE temP_x SET c_amount = 1.22 WHERE c_id = 2;

# 报错 不可执行
UPDATE temp_x a
    SET a.`c_amount` = b.c_amount
    FROM temp_y b
    WHERE a.`c_id` = b.c_id;

# 还是报错
UPDATE temp_x a
SET a.`c_amount` = b.`c_amount`
FROM temp_x a, temp_y b
WHERE a.`c_id` = b.`c_id`;    

　　可行的办法：

# 方法一 可行
UPDATE temp_x a, temp_y b
SET a.`c_amount` = b.`c_amount`
WHERE a.`c_id` = b.`c_id`;

# 方法二 可行
UPDATE temp_x a 
    SET a.`c_amount` = (SELECT b.c_amount FROM temp_y b
                WHERE b.`c_id` = a.`c_id`)

# 方法三 可行 方法二加强版
UPDATE temp_x a
SET a.c_amount = (SELECT b.c_amount
            FROM temp_y b
          WHERE b.c_id = a.c_id)
WHERE a.c_id IN (SELECT b.c_id FROM temp_y b);

　　其他例子：

# update 一列
UPDATE student s, city c
    SET s.city_name = c.name
WHERE s.city_code = c.code;

# update 多列
UPDATE a, b
    SET a.title = b.title
        a.name = b.name
WHERE a.id = b.id;

# 通过子查询
UPDATE student s
SET s.city_name =
    (SELECT NAME FROM city
      WHERE CODE = s.city_code);

# 复杂查询
UPDATE a SET
xx = (SELECT b.xxx FROM b WHERE b.id = a.id),
yy = (SELECT b.xxx FROM b WHERE b.id = a.id)
WHERE EXISTS(SELECT b.xxx FROM b WHERE b.id = a.id);

# 优化写法
UPDATE a INNER JOIN b ON a.id = b.id 
SET a.xx = b.xxx
    a.yy = b.xxx;

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 END 2018-05-29 17:01:00