Question

我有两个包含（ID，值）的表。可以在任一表或两个表中找到ID。我需要一个包含所有唯一ID及其值的记录集;如果ID出现在两个表中，我需要表2中的值。

目前我有这个问题：

SELECT * FROM
(SELECT * FROM table2 UNION SELECT * FROM table1) as temp
GROUP BY id

似乎有用。但是 -

如果ID在两个表中，保证是否从table2返回值？
如果没有，我应该怎么做？
无论如何，有更好的方法吗？

Answer 1

尝试一下，

SELECT  a.*
FROM    
        (
            SELECT *, 1 AS TB FROM table1  -- assuming you have many columns
            UNION ALL
            SELECT *, 2 AS TB FROM table2
        ) a INNER JOIN
        (
            SELECT ID, MAX(TB) maxTB
            FROM 
            (
                SELECT ID, 1 AS TB FROM table1
                UNION ALL
                SELECT ID, 2 AS TB FROM table2
            ) s
            GROUP BY ID
        ) b ON  a.ID = b.ID AND 
                a.tb = b.maxTB

SQLFiddle Demo

Answer 2

把我的两分钱放在

SELECT ISNULL(TB2.Id, TB1.Id) As Id,
       ChooseValue(TB2.Id, TB2.Value, TB1.Values) As Value
FROM TABLE2 AS TB2
FULL OUTER JOIN TABLE1 AS TB1
ON TB2.Id = TB1.Id

这可以避免额外加入或额外运行表或group by子句的成本，这可能很昂贵。

CREATE FUNCTION ChooseValue(DeciderValue IN VAR_TYPE, Table_2Value IN VARTYPE, Table_1Value  IN VARTYPE)
RETURNS VARTYPE
BEGIN
    DECLARE returnValue VARTYPE;
    IF DeciderValue IS NULL THEN
        returnValue = Table_2Value
    ELSE
        returnValue = Table_1Value
    END IF
RETURN returnValue
END

MySQL UNION查询，不同的ID

2 个答案: