一、多表連接查詢

1 交叉連接：不適用任何匹配條件。生成笛卡爾積

mysql> select * fromemployee,department;+----+------------+--------+------+--------+------+--------------+

| id | name | sex | age | dep_id | id | name |

+----+------------+--------+------+--------+------+--------------+

| 1 | egon | male | 18 | 200 | 200 | 技術 |

| 1 | egon | male | 18 | 200 | 201 | 人力資源 |

| 1 | egon | male | 18 | 200 | 202 | 銷售 |

| 1 | egon | male | 18 | 200 | 203 | 運營 |

| 2 | alex | female | 48 | 201 | 200 | 技術 |

| 2 | alex | female | 48 | 201 | 201 | 人力資源 |

| 2 | alex | female | 48 | 201 | 202 | 銷售 |

| 2 | alex | female | 48 | 201 | 203 | 運營 |

| 3 | wupeiqi | male | 38 | 201 | 200 | 技術 |

| 3 | wupeiqi | male | 38 | 201 | 201 | 人力資源 |

| 3 | wupeiqi | male | 38 | 201 | 202 | 銷售 |

| 3 | wupeiqi | male | 38 | 201 | 203 | 運營 |

| 4 | yuanhao | female | 28 | 202 | 200 | 技術 |

| 4 | yuanhao | female | 28 | 202 | 201 | 人力資源 |

| 4 | yuanhao | female | 28 | 202 | 202 | 銷售 |

| 4 | yuanhao | female | 28 | 202 | 203 | 運營 |

| 5 | liwenzhou | male | 18 | 200 | 200 | 技術 |

| 5 | liwenzhou | male | 18 | 200 | 201 | 人力資源 |

| 5 | liwenzhou | male | 18 | 200 | 202 | 銷售 |

| 5 | liwenzhou | male | 18 | 200 | 203 | 運營 |

| 6 | jingliyang | female | 18 | 204 | 200 | 技術 |

| 6 | jingliyang | female | 18 | 204 | 201 | 人力資源 |

| 6 | jingliyang | female | 18 | 204 | 202 | 銷售 |

| 6 | jingliyang | female | 18 | 204 | 203 | 運營 |

+----+------------+--------+------+--------+------+--------------+

2 內連接：只連接匹配的行

#找兩張表共有的部分，相當于利用條件從笛卡爾積結果中篩選出了正確的結果#department沒有204這個部門，因而employee表中關于204這條員工信息沒有匹配出來

mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee inner join department on employee.dep_id=department.id;+----+-----------+------+--------+--------------+

| id | name | age | sex | name |

+----+-----------+------+--------+--------------+

| 1 | egon | 18 | male | 技術 |

| 2 | alex | 48 | female | 人力資源 |

| 3 | wupeiqi | 38 | male | 人力資源 |

| 4 | yuanhao | 28 | female | 銷售 |

| 5 | liwenzhou | 18 | male | 技術 |

+----+-----------+------+--------+--------------+

#上述sql等同于

mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee,department where employee.dep_id=department.id;

3 外鏈接之左連接：優先顯示左表全部記錄

#以左表為準，即找出所有員工信息，當然包括沒有部門的員工#本質就是：在內連接的基礎上增加左邊有右邊沒有的結果

mysql> select employee.id,employee.name,department.name as depart_name from employee left join department on employee.dep_id=department.id;+----+------------+--------------+

| id | name | depart_name |

+----+------------+--------------+

| 1 | egon | 技術 |

| 5 | liwenzhou | 技術 |

| 2 | alex | 人力資源 |

| 3 | wupeiqi | 人力資源 |

| 4 | yuanhao | 銷售 |

| 6 | jingliyang | NULL |

+----+------------+--------------+

4 外鏈接之右連接：優先顯示右表全部記錄

#以右表為準，即找出所有部門信息，包括沒有員工的部門#本質就是：在內連接的基礎上增加右邊有左邊沒有的結果

mysql> select employee.id,employee.name,department.name as depart_name from employee right join department on employee.dep_id=department.id;+------+-----------+--------------+

| id | name | depart_name |

+------+-----------+--------------+

| 1 | egon | 技術 |

| 2 | alex | 人力資源 |

| 3 | wupeiqi | 人力資源 |

| 4 | yuanhao | 銷售 |

| 5 | liwenzhou | 技術 |

| NULL | NULL | 運營 |

+------+-----------+--------------+

5 全外連接：顯示左右兩個表全部記錄

全外連接：在內連接的基礎上增加左邊有右邊沒有的和右邊有左邊沒有的結果#注意：mysql不支持全外連接 full JOIN#強調：mysql可以使用此種方式間接實現全外連接

select * from employee left join department on employee.dep_id =department.id

union

select* from employee right join department on employee.dep_id =department.id

;#查看結果

+------+------------+--------+------+--------+------+--------------+

| id | name | sex | age | dep_id | id | name |

+------+------------+--------+------+--------+------+--------------+

| 1 | egon | male | 18 | 200 | 200 | 技術 |

| 5 | liwenzhou | male | 18 | 200 | 200 | 技術 |

| 2 | alex | female | 48 | 201 | 201 | 人力資源 |

| 3 | wupeiqi | male | 38 | 201 | 201 | 人力資源 |

| 4 | yuanhao | female | 28 | 202 | 202 | 銷售 |

| 6 | jingliyang | female | 18 | 204 | NULL | NULL |

| NULL | NULL | NULL | NULL | NULL | 203 | 運營 |

+------+------------+--------+------+--------+------+--------------+

#注意 union與union all的區別：union會去掉相同的紀錄

二、子查詢

1：子查詢是將一個查詢語句嵌套在另一個查詢語句中。

2：內層查詢語句的查詢結果，可以為外層查詢語句提供查詢條件。

3：子查詢中可以包含：IN、NOT IN、ANY、ALL、EXISTS 和 NOT EXISTS等關鍵字

4：還可以包含比較運算符：= 、 !=、> 、

1 帶IN關鍵字的子查詢

查詢employee表，但dep_id必須在department表中出現過

select* fromemployee

where dep_idin(select idfrom department);

2 帶比較運算符的子查詢

#比較運算符：=、!=、>、>=、#查詢平均年齡在25歲以上的部門名

select id,name fromdepartment

where idin(select dep_idfrom employee group by dep_id having avg(age) > 25);#查看技術部員工姓名

select name fromemployee

where dep_idin(select idfrom department where name='技術');#查看不足1人的部門名

select name fromdepartment

where idin(select dep_idfrom employee group by dep_id having count(id) <=1);

3 帶EXISTS關鍵字的子查詢

EXISTS關字鍵字表示存在。在使用EXISTS關鍵字時，內層查詢語句不返回查詢的記錄。

而是返回一個真假值。True或False

當返回True時，外層查詢語句將進行查詢；當返回值為False時，外層查詢語句不進行查詢

#department表中存在dept_id=203，Ture

mysql> select * fromemployee->where exists-> (select id from department where id=200);+----+------------+--------+------+--------+

| id | name | sex | age | dep_id |

+----+------------+--------+------+--------+

| 1 | egon | male | 18 | 200 |

| 2 | alex | female | 48 | 201 |

| 3 | wupeiqi | male | 38 | 201 |

| 4 | yuanhao | female | 28 | 202 |

| 5 | liwenzhou | male | 18 | 200 |

| 6 | jingliyang | female | 18 | 204 |

+----+------------+--------+------+--------+

#department表中存在dept_id=205，False

mysql> select * fromemployee->where exists-> (select id from department where id=204);

Empty set (0.00 sec)

總結

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