MySQL 連接查詢、聯合查詢和子查詢

準備測試數據

create table class(
	cid int(10),
	cdesc varchar(20)
);

create table student(
	sid int(10),
	name varchar(20),
	age int(3),
	cid int(10)
);

create table teacher(
	tid int(10),
	name varchar(20),
	cid int(10)
);

insert into class values(1, "PHP"),(2, "Java"),(3, "C++"),(4,"SQL");
insert into student values(1, "s1",16,1),(2, "s2",17,2),(3, "s3",18,3),(4,"s4",19,4),(5, "s5",18,3),(6,"s6",19,4);

insert into teacher values(1, "t1",1),(2, "t2",2),(3, "t3",3),(4,"t4",4);

alter table student add constraint sid_pk primary key(sid);

連接查詢 join

連接查詢方式有：內連接、外連接（常用，分爲左連接和右連接）、自然連接、交叉連接。藉助連接查詢，可以同時查看多張表中的數據。

• 內連接：有條件連接，用 on 在多個表之間指定條件連接。如果兩個表的任一個匹配不到數據，則返回空
• 外連接：有條件連接，用 on 在多個表之間指定條件連接。只要主表（左連接的左表，右連接的右表）匹配到數據，就不會返回空，未匹配到數據的字段填充 NULL

mysql> select * from class join student on student.cid = class.cid where class.cid = 1;
Empty set (0.00 sec)

mysql> select * from class left join student on student.cid = class.cid where class.cid = 1;
+-----+-------+------+------+------+------+
| cid | cdesc | sid  | name | age  | cid  |
+-----+-------+------+------+------+------+
|   1 | PHP   | NULL | NULL | NULL | NULL |
+-----+-------+------+------+------+------+
1 row in set (0.00 sec)

mysql> select * from student left join class on student.cid = class.cid where class.cid = 1;
Empty set (0.00 sec)

語法

• 內連接：select 字段列表 from 左表 [inner] join 右表 on 左表.字段 = 右表.字段;
• 外連接（重要）：
  左外連接：select 字段列表 from 左表 left join 右表 on 左表.字段 = 右表.字段;
  右外連接：select 字段列表 from 左表 right join 右表 on 左表.字段 = 右表.字段;
• 自然連接：有條件連接，但是不需要指定，MySQL 自動依據“同名字段”連接（多個同名字段就都作爲條件）。
• 交叉連接 cross join：無條件連接，取笛卡爾積，返回的記錄數等於各表記錄數的乘積。

聯合查詢 union

將多個查詢結果進行拼接，多個查詢結果的字段數必須相同，類型可以不同。支持兩個選項：

• all：不對相同結果去重，默認選項
• distinct：去重

mysql> select age from student union distinct select cdesc from class;
+------+
| age  |
+------+
| 17   |
| 18   |
| 19   |
| C++  |
| Java |
| PHP  |
| SQL  |
+------+
7 rows in set (0.00 sec)

子查詢

• from 子查詢：from 後面是子查詢。注意這個子查詢會創建臨時表，需要爲其起個別名：

mysql> select sid from (select * from student where sid > 2);
ERROR 1248 (42000): Every derived table must have its own alias
mysql> select sid from (select * from student where sid > 2) as s;
+-----+
| sid |
+-----+
|   3 |
|   4 |
|   5 |
|   6 |
+-----+
4 rows in set (0.00 sec)

• where 子查詢：where 後面是子查詢。

mysql> select * from student where age > (select age from student where sid = 3);
+-----+------+------+------+
| sid | name | age  | cid  |
+-----+------+------+------+
|   4 | s4   |   19 |    4 |
|   6 | s6   |   19 |    4 |
+-----+------+------+------+
2 rows in set (0.00 sec)

• where exists 子查詢：where exists 後面是子查詢。

mysql> select * from class where exists(select * from  student where cid=1);
Empty set (0.00 sec)

mysql> select * from class where exists(select * from  student where cid=2);
+-----+-------+
| cid | cdesc |
+-----+-------+
|   1 | PHP   |
|   2 | Java  |
|   3 | C++   |
|   4 | SQL   |
+-----+-------+
4 rows in set (0.00 sec)