[英]Joining two tables based on column value
如果第一个连接满足条件,我需要连接不同的表,因此,如果第一个连接的列的值为 1,则与一个表进行连接,但如果值为 2,则与另一个表进行连接,像这样,但 MySQL 给出错误
SELECT col1, col2, col3,...,col8
FROM table1 AS tb1
INNER JOIN table2 AS t2 ON tb1.id = tb2.id
INNER JOIN table3 AS t3 ON tb2.id = tb3.id
CASE
WHEN tb2.col2 = 1 THEN INNER JOIN table4 ON id = col1
WHEN tb2.col2 = 2 THEN INNER JOIN table5 ON id = col3
WHEN tb2.col2 = 3 THEN INNER JOIN table6 ON id = col5
END
WHERE tb1.id = 13;
case
表达式返回一个值,不能用于代码的条件执行。 做LEFT JOIN
代替。
SELECT col1, col2, col3,...,col8
FROM table1 AS tb1
INNER JOIN table2 AS t2 ON tb1.id = tb2.id
INNER JOIN table3 AS t3 ON tb2.id = tb3.id
LEFT JOIN table4 ON id = col1 and tb2.col2 = 1
LEFT JOIN table5 ON id = col3 and tb2.col2 = 2
LEFT JOIN table6 ON id = col5 and tb2.col2 = 3
WHERE tb1.id = 13;
您可以使用 LEFT 连接并在 WHERE 子句中进行过滤:
SELECT col1, col2, col3,...,col8
FROM table1 AS tb1
INNER JOIN table2 AS tb2 ON tb1.id = tb2.id
INNER JOIN table3 AS tb3 ON tb2.id = tb3.id
LEFT JOIN table4 ON id = col1 AND tb2.col2 = 1
LEFT JOIN table5 ON id = col3 AND tb2.col2 = 2
LEFT JOIN table6 ON id = col5 AND tb2.col2 = 3
WHERE tb1.id = 13 AND tb2.col2 IN (1, 2, 3);
请注意,所有列都应使用相应的表名/别名进行限定。
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