SQL

连接

在 MySQL 中 JOIN、 CROSS JOIN 、 INNER JOIN 是等价的，都是内连接。

内连接

• JOIN、 CROSS JOIN 、 INNER JOIN 当使用这三个子句时，其结果都是笛卡尔积；

• 如果以上三个子句加上 ON，则为 等值连接：只会返回 ON 子句相等的结果

外连接

外连接分为 LEFT JOIN 、 RIGHT JOIN 和 NATURAL JOIN，所有外连接均可省略 OUTER 关键字，即 LEFT OUTER JOIN...ON... 与 LEFT JOIN...ON...等效。

• T1 LEFT JOIN T2 ON T1.id=T2.id：左外连接，返回所有列、T1 所有行、T2 中 条件符合的行

• T1 RIGHT JOIN T2 ON T1.id=T2.id：右外连接，返回所有列、T2 所有行、T1 中 条件符合的行

• T1 NATURAL JOIN T2：自然连接，返回 T1 所有行、T2 中与 T1 匹配的行，相同的属性被合并

对于自然连接要多做说明，现在有表 join_test1 、join_test2：

mysql> select * from  join_test1;
+----+------+
| id | name |
+----+------+
|  1 | A    |
|  2 | B    |
|  3 | C    |
+----+------+
3 rows in set (0.00 sec)

mysql> select * from  join_test2;
+----+------+
| id | sex  |
+----+------+
|  1 | 男   |
|  2 | 女   |
|  4 | 男   |
+----+------+
3 rows in set (0.00 sec)

自然连接的结果：

mysql> select * from join_test1 NATURAL join join_test2;
+----+------+------+
| id | name | sex  |
+----+------+------+
|  1 | A    | 男   |
|  2 | B    | 女   |
+----+------+------+

上面三种子句，又可组合出 NATURAL LEFT|RIGHT JOIN...：这种子句就结合了 NATURAL JOIN 和 LEFT|RIGHT JOIN..ON... 的特点：在执行左|右连接的同时，将相同属性合并。

mysql> select * from join_test1 NATURAL left join join_test2;
+----+------+------+
| id | name | sex  |
+----+------+------+
|  1 | A    | 男   |
|  2 | B    | 女   |
|  3 | C    | NULL |
+----+------+------+
3 rows in set (0.00 sec)

子查询

子查询作为标量

SELECT (SELECT s2 FROM t1);

子查询比较

non_subquery_operand comparison_operator (subquery)

= > < >= <= <> != <=> LIKE

ANY IN SOME

operand comparison_operator ANY (subquery)
operand IN (subquery)
operand comparison_operator SOME (subquery)

EXISTS or NOT EXISTS

SELECT column1 FROM t1 WHERE EXISTS (SELECT * FROM t2);

ALL

operand comparison_operator ALL (subquery)

SELECT s1 FROM t1 WHERE s1 > ALL (SELECT s1 FROM t2);

关联查询

SELECT * FROM t1
  WHERE column1 = ANY (SELECT column1 FROM t2
                       WHERE t2.column2 = t1.column2);

select Score, (select count(distinct Score) from Scores b where b.Score>= s.Score) Rank from Scores s order by Score desc

派生表

SELECT ... FROM (subquery) [AS] tbl_name ...