条件连接(join) T1 { [INNER] | { LEFT | RIGHT | FULL } [OUTER] } JOIN T2 ON boolean_expression T1 { [INNER] | { LEFT | RIGHT | FULL } [OUTER] } JOIN T2 USING ( join column list ) T1 NATURAL { [INNER] | { LEFT | RIGHT | FULL } [OUTER] } JOIN T2 INNER 和 OUTER 对所有连接(join) 类型都是可选的.INNER 是缺省; LEFT,RIGHT,和 FULL 隐含外连接.
num | value -----+------- 1 | xxx 3 | yyy 5 | zzz 然后我们用不同的连接方式可以获得各种结果:
=> SELECT * FROM t1 CROSS JOIN t2; num | name | num | value -----+------+-----+------- 1 | a | 1 | xxx 1 | a | 3 | yyy 1 | a | 5 | zzz 2 | b | 1 | xxx 2 | b | 3 | yyy 2 | b | 5 | zzz 3 | c | 1 | xxx 3 | c | 3 | yyy 3 | c | 5 | zzz (9 rows)
=> SELECT * FROM t1 INNER JOIN t2 ON t1.num = t2.num; num | name | num | value -----+------+-----+------- 1 | a | 1 | xxx 3 | c | 3 | yyy (2 rows)
=> SELECT * FROM t1 INNER JOIN t2 USING (num); num | name | value -----+------+------- 1 | a | xxx 3 | c | yyy (2 rows)
=> SELECT * FROM t1 NATURAL INNER JOIN t2; num | name | value -----+------+------- 1 | a | xxx 3 | c | yyy (2 rows)
=> SELECT * FROM t1 LEFT JOIN t2 ON t1.num = t2.num; num | name | num | value -----+------+-----+------- 1 | a | 1 | xxx 2 | b | | 3 | c | 3 | yyy (3 rows)
=> SELECT * FROM t1 LEFT JOIN t2 USING (num); num | name | value -----+------+------- 1 | a | xxx 2 | b | 3 | c | yyy (3 rows)
=> SELECT * FROM t1 RIGHT JOIN t2 ON t1.num = t2.num; num | name | num | value -----+------+-----+------- 1 | a | 1 | xxx 3 | c | 3 | yyy | | 5 | zzz (3 rows)
=> SELECT * FROM t1 FULL JOIN t2 ON t1.num = t2.num; num | name | num | value -----+------+-----+------- 1 | a | 1 | xxx 2 | b | | 3 | c | 3 | yyy | | 5 | zzz (4 rows)
用 ON 声明的连接条件也可以包含与连接不直接相关 的条件。这种功能可能对某些查询很有用,但是需要我们仔细想想。 比如:
=> SELECT * FROM t1 LEFT JOIN t2 ON t1.num = t2.num AND t2.value = 'xxx'; num | name | num | value -----+------+-----+------- 1 | a | 1 | xxx 2 | b | | 3 | c | | (3 rows)