1. inner join 内连接查询
SELECT a.,b. FROM table_a a INNER JOIN table_b b ON a.id=b.id；
2. left join 左关联查询
SELECT a.,b. FROM table_a a LEFT JOIN table_b b ON a.id=b.id；
3.right join 右关联查询
SELECT a.,b. FROM table_a a RIGHT JOIN table_b b ON a.id=b.id；
4.左连接-内连接
SELECT a.,b. FROM table_a a LEFT JOIN table_b b ON a.id=b.id WHERE b.id IS NULL；
5. 右连接-内连接
SELECT a.,b. FROM table_a a RIGHT JOIN table_b b ON a.id=b.id WHERE a.id IS NULL
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