[英]MySQL JOIN if either of the 2 tables meets condition
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)。
仅当table1.value = table2.value
或table1.value = table3.value
为true时,如何才能进行连接?
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)
您可以使用两个查询和UNION
它们,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)
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
Inner 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。
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