Question

我需要加入两对表。如果表1中的ID也可以在表3中找到，我需要加入表。如果表3中的Table1没有匹配的ID，我不需要加入表。

实施例

如果表3中的表1中至少有一个id;做一些有效的事情：


    SELECT *
    FROM Table1 AS t1
    INNER JOIN Table2 AS t2 ON t1.ID = t2.ID
    LEFT JOIN Table3 AS t3 ON t1.ID = t3.ID
    LEFT JOIN Table4 AS t4 ON t3.ID = t4.ID

如果Table1和Table3之间没有匹配的ID;做一些有效的事情：


    SELECT *
    FROM Table1 AS t1
    INNER JOIN Table2 AS t2 ON t1.ID = t2.ID

Answer 1

只需将您的问题翻译成SQL，即可：

IF EXISTS(SELECT * FROM Table1 T1 INNER JOIN Table3 T3 ON T1.ID=T3.ID)
    SELECT *
    FROM Table1 AS t1
    INNER JOIN Table2 AS t2 ON t1.ID = t2.ID
    LEFT JOIN Table3 AS t3 ON t1.ID = t3.ID
    LEFT JOIN Table4 AS t4 ON t3.ID = t4.ID
ELSE  
    SELECT *
    FROM Table1 AS t1
    INNER JOIN Table2 AS t2 ON t1.ID = t2.ID

左加入;仅当至少有一个匹配记录时

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