根据列值连接两个表

Question

如果在第一次连接时满足条件，我需要连接不同的表，因此，如果第一次连接的列的值为 1，则与一个表进行连接，但如果值为 2，则使用另一个表，类似这样，但 MySQL 报错

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;

Answer 1

case表达式返回一个值，不能用于条件执行代码。改为LEFT JOINs。

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;

Answer 2

您可以在 WHERE 子句中使用 LEFT 连接和过滤器：

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。