Question

SELECT COALESCE(table2.value, table3.value) FROM table1
JOIN table2 ON (table1.value = table2.value)
JOIN table3 ON (table1.value = table3.value)

table1.value = table2.value和table1.value = table3.value是互斥的（最多其中一个是真的）。

如果table1.value = table2.value或table1.value = table3.value为真，我如何才能进行加入？

这是唯一的解决方案吗？（这很乏味）

SELECT table.value FROM table1
JOIN 
(SELECT COALESCE(table2.value, table3.value) FROM table1
 LEFT JOIN table2 ON (table1.value = table2.value)
 LEFT JOIN table3 ON (table1.value = table3.value)
) as table ON (table1.value = table.value)

Answer 1

您可以使用两个查询和UNION个查询，例如：

SELECT COALESCE(table1.value, table2.value) FROM table1
JOIN table2 ON (table1.value = table2.value)

UNION

SELECT COALESCE(table1.value, table3.value) FROM table1
JOIN table3 ON (table1.value = table3.value)

Answer 2

尝试使用内连接和联合两个表

SELECT * FROM
(SELECT value, data FROM table1) as a
INNER JOIN
(SELECT value, data FROM table2) as b
ON b.value = a.value
UNION ALL
(SELECT value, data FROM table1) as a
INNER JOIN
(SELECT value, data FROM table3) as b
ON b.value = a.value

内部JOIN将避免连接两个为NULL的表，它只会加入那些在两个表上都有完全查找的行。在UNION中我们假设在第一个JOIN中，另一个table3将不会被包含在那里，所以将它作为单独的连接和UNION它或将它合并到第一个，这样你就可以在一个查询/表上获得所有数据只要记住使用UNION，所有结果都应该具有您想要获取的确切名称或字段。

如果两个表中的任何一个满足条件，则MySQL JOIN

2 个答案: