[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(最多其中一個是真的)。
僅當table1.value = table2.value
或table1.value = table3.value
為true時,如何才能進行連接?
這是唯一的解決方案嗎? (這很乏味)
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)
您可以使用兩個查詢和UNION
它們,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)
嘗試使用內連接和聯合兩個表
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將避免連接兩個為NULL的表,它只會加入那些在兩個表上都有完全查找的行。 在UNION中我們假設在第一個JOIN中,另一個table3將不會被包含在那里,所以將它作為單獨的連接和UNION它或將它合並到第一個,這樣你就可以在一個查詢/表上獲得所有數據只要記住使用UNION,所有結果都應該具有您想要獲取的確切名稱或字段。
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