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MySQL 后from多个表_MYSQL回顾（多表查询相关）

發(fā)布時間：2023/12/1 数据库 26 豆豆

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前言

簡單的數(shù)據(jù)我們可以直接從一個表中獲取，但在真實的項目中查詢符合條件的數(shù)據(jù)通常需要牽扯到多張表，這就不得不使用多表查詢。多表查詢分為多表連接查詢、符合條件鏈接查詢、子查詢。多表連接查詢包括內(nèi)連接、外連接、全連接。符合條件連接查詢本質(zhì)上是多表連接查詢+過濾條件。子查詢是將一個查詢語句嵌套在另一個查詢語句中，內(nèi)層查詢語句的查詢結(jié)果作為外層查詢語句的數(shù)據(jù)源。

準(zhǔn)備

# 建表

create table department(

id int,

name varchar(20)

);

create table employee(

id int primary key auto_increment,

name varchar(20),

sex enum('male','female') not null default 'male',

age int,

dep_id int

);

# 插入數(shù)據(jù)

insert into department values

(1,'技術(shù)'),

(2,'財務(wù)'),

(3,'法律’);

# 此處省略員工表數(shù)據(jù)...

多表連接查詢

語法

SELECT 字段列表

FROM 表1 INNER|LEFT|RIGHT JOIN 表2

ON 表1.字段 = 表2.字段;

交叉連接

在介紹多表查詢的時候，有必要先介紹下交叉連接，如下select * from employee, department;查詢語句就是交叉連接查詢，可以看出，同一個數(shù)據(jù)在在employee表和department表交叉連接之后產(chǎn)生了重復(fù)記錄，其重復(fù)個數(shù)取決于department表的記錄個數(shù)。所以最后交叉連接之后的記錄個數(shù)是：count(employee) * count(department)，即笛卡爾積。通常情況下，笛卡爾積的結(jié)果在工作中無實際意義，我們需要在笛卡爾積的基礎(chǔ)上進行篩選，找到employee.dep_id = department.id的那條記錄。

mysql> select * from employee, department;

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

| id | name | age | sex | position | salary | dep_id | id | name |

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

| 1 | jack | 20 | male | lawyer | 888889 | 3 | 1 | 技術(shù) |

| 1 | jack | 20 | male | lawyer | 888889 | 3 | 2 | 財務(wù) |

| 1 | jack | 20 | male | lawyer | 888889 | 3 | 3 | 法律 |

| 2 | mark | 22 | male | lawyer | 888889 | 3 | 1 | 技術(shù) |

| 2 | mark | 22 | male | lawyer | 888889 | 3 | 2 | 財務(wù) |

| 2 | mark | 22 | male | lawyer | 888889 | 3 | 3 | 法律 |

| 3 | hank | 25 | male | lawyer | 7777.8 | 3 | 1 | 技術(shù) |

| 3 | hank | 25 | male | lawyer | 7777.8 | 3 | 2 | 財務(wù) |

| 3 | hank | 25 | male | lawyer | 7777.8 | 3 | 3 | 法律 |

| 4 | nick | 39 | male | lawyer | 4438890 | 3 | 1 | 技術(shù) |

| 4 | nick | 39 | male | lawyer | 4438890 | 3 | 2 | 財務(wù) |

| 4 | nick | 39 | male | lawyer | 4438890 | 3 | 3 | 法律 |

| 5 | jenny | 26 | female | lawyer | 10000.8 | 3 | 1 | 技術(shù) |

| 5 | jenny | 26 | female | lawyer | 10000.8 | 3 | 2 | 財務(wù) |

| 5 | jenny | 26 | female | lawyer | 10000.8 | 3 | 3 | 法律 |

| 6 | tony | 35 | male | RD | 100000000 | 1 | 1 | 技術(shù) |

| 6 | tony | 35 | male | RD | 100000000 | 1 | 2 | 財務(wù) |

| 6 | tony | 35 | male | RD | 100000000 | 1 | 3 | 法律 |

| 7 | emmy | 27 | female | RD | 9999 | 1 | 1 | 技術(shù) |

| 7 | emmy | 27 | female | RD | 9999 | 1 | 2 | 財務(wù) |

| 7 | emmy | 27 | female | RD | 9999 | 1 | 3 | 法律 |

| 8 | emmy | 23 | female | finance | 5000 | 2 | 1 | 技術(shù) |

| 8 | emmy | 23 | female | finance | 5000 | 2 | 2 | 財務(wù) |

| 8 | emmy | 23 | female | finance | 5000 | 2 | 3 | 法律 |

| 9 | lucy | 45 | female | finance | 10000 | 2 | 1 | 技術(shù) |

| 9 | lucy | 45 | female | finance | 10000 | 2 | 2 | 財務(wù) |

| 9 | lucy | 45 | female | finance | 10000 | 2 | 3 | 法律 |

| 10 | emmi | 20 | female | finance | 20000 | 2 | 1 | 技術(shù) |

| 10 | emmi | 20 | female | finance | 20000 | 2 | 2 | 財務(wù) |

| 10 | emmi | 20 | female | finance | 20000 | 2 | 3 | 法律 |

| 11 | james | 18 | male | NULL | 3000 | NULL | 1 | 技術(shù) |

| 11 | james | 18 | male | NULL | 3000 | NULL | 2 | 財務(wù) |

| 11 | james | 18 | male | NULL | 3000 | NULL | 3 | 法律 |

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

33 rows in set (0.00 sec)

內(nèi)連接

內(nèi)連接只會連接兩張表匹配的行，即取交集。找兩張表公共部分，相當(dāng)于利用條件從笛卡爾積結(jié)果中篩選出了正確的結(jié)果

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 | jack | 20 | male | 法律 |

| 2 | mark | 22 | male | 法律 |

| 3 | hank | 25 | male | 法律 |

| 4 | nick | 39 | male | 法律 |

| 5 | jenny | 26 | female | 法律 |

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

10 rows in set (0.00 sec)

上述內(nèi)連接查詢語句等同于：

mysql> select employee.id,employee.name,employee.age,employee.sex,department.name

from employee,department

where employee.dep_id=department.id;

外連接

外連接分為左連接、右連接、全外連接

左連接

左連接在內(nèi)連接的基礎(chǔ)上優(yōu)先顯示左表全部記錄。即左連接=內(nèi)連接+左表未符合條件的記錄

#以左表為準(zhǔn)，即找出所有員工信息，當(dāng)然包括沒有部門的員工

#本質(zhì)就是：在內(nèi)連接的基礎(chǔ)上增加左邊有右邊沒有的結(jié)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 |

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

| 6 | tony | 技術(shù) |

| 7 | emmy | 技術(shù) |

| 8 | emmy | 財務(wù) |

| 9 | lucy | 財務(wù) |

| 10 | emmi | 財務(wù) |

| 1 | jack | 法律 |

| 2 | mark | 法律 |

| 3 | hank | 法律 |

| 4 | nick | 法律 |

| 5 | jenny | 法律 |

| 11 | james | NULL |

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

11 rows in set (0.00 sec)

右連接

又連接在內(nèi)連接的基礎(chǔ)上優(yōu)先顯示右表的內(nèi)容。即右連接==內(nèi)連接+右表未符合條件的記錄

#以右表為準(zhǔn)，即找出所有部門信息，包括沒有員工的部門

#本質(zhì)就是：在內(nèi)連接的基礎(chǔ)上增加右邊有左邊沒有的結(jié)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 | jack | 法律 |

| 2 | mark | 法律 |

| 3 | hank | 法律 |

| 4 | nick | 法律 |

| 5 | jenny | 法律 |

| 6 | tony | 技術(shù) |

| 7 | emmy | 技術(shù) |

| 8 | emmy | 財務(wù) |

| 9 | lucy | 財務(wù) |

| 10 | emmi | 財務(wù) |

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

10 rows in set (0.00 sec)

可以發(fā)現(xiàn)，左表(employee表)的第11條記錄沒有被查詢出來

全外連接

全外連接會在內(nèi)連接查詢的基礎(chǔ)上顯示左表和右表的全部記錄

mysql> 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

-> ;

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

| id | name | age | sex | position | salary | dep_id | id | name |

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

| 6 | tony | 35 | male | RD | 100000000 | 1 | 1 | 技術(shù) |

| 7 | emmy | 27 | female | RD | 9999 | 1 | 1 | 技術(shù) |

| 8 | emmy | 23 | female | finance | 5000 | 2 | 2 | 財務(wù) |

| 9 | lucy | 45 | female | finance | 10000 | 2 | 2 | 財務(wù) |

| 10 | emmi | 20 | female | finance | 20000 | 2 | 2 | 財務(wù) |

| 1 | jack | 20 | male | lawyer | 888889 | 3 | 3 | 法律 |

| 2 | mark | 22 | male | lawyer | 888889 | 3 | 3 | 法律 |

| 3 | hank | 25 | male | lawyer | 7777.8 | 3 | 3 | 法律 |

| 4 | nick | 39 | male | lawyer | 4438890 | 3 | 3 | 法律 |

| 5 | jenny | 26 | female | lawyer | 10000.8 | 3 | 3 | 法律 |

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

12 rows in set (0.00 sec)

符合條件鏈接查詢

mysql> select employee.name, employee.age, department.name from employee inner join department

-> on employee.dep_id=department.id

-> where age > 30

-> order by age asc;

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

| name | age | name |

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

| tony | 35 | 技術(shù) |

| nick | 39 | 法律 |

| lucy | 45 | 財務(wù) |

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

3 rows in set (0.00 sec)

子查詢

子查詢是將一個查詢語句的嵌套在另一個查詢語句中

內(nèi)層查詢語句的查詢結(jié)果作為外層查詢語句的數(shù)據(jù)源

子查詢中可以包含 IN、NOT IN、ANY、ALL、EXISTS和NOT EXISTS等關(guān)鍵字

注意被嵌套的查詢語句需要用( )包裹

查詢員工部門平均年齡大于30的部門信息

mysql> select * from department

-> where id in

-> (select dep_id from employee group by dep_id having avg(age) > 30);

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

| id | name |

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

| 1 | 技術(shù) |

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

1 row in set (0.00 sec)

查詢技術(shù)部員工姓名

mysql> select name from employee

-> where dep_id in

-> (select id from department where name="技術(shù)");

+------+

| name |

+------+

| tony |

| emmy |

+------+

2 rows in set (0.00 sec)

查詢無員工的部門名(子查詢得到的是所有人的部門id，需要disctinct去除)

mysql> select name from department

-> where id not in

-> (select distinct dep_id from employee);

帶比較運算符的子查詢

查詢大于所有人平均年齡的員工名和年齡

mysql> select * from employee where age > (select avg(age) from employee);

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

| id | name | age | sex | position | salary | dep_id |

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

| 4 | nick | 39 | male | lawyer | 4438890 | 3 |

| 6 | tony | 35 | male | RD | 100000000 | 1 |

| 9 | lucy | 45 | female | finance | 10000 | 2 |

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

3 rows in set (0.00 sec)

不能這樣：在前面沒有g(shù)roup by的時后面不能使用分組函數(shù)

mysql> select * from employee where age > avg(age);

ERROR 1111 (HY000): Invalid use of group function

帶EXISTS關(guān)鍵字的子查詢

EXISTS關(guān)字鍵字表示存在。在使用EXISTS關(guān)鍵字時，內(nèi)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(dāng)返回True時，外層查詢語句將進行查詢；當(dāng)返回值為False時，外層查詢語句不進行查詢

mysql> select * from employee

-> where exists

-> (select id from department where id=200);

MYSQL中可以把一個查詢語句用括號括起來使用as起一個別名當(dāng)做一個表使用

查詢每個職位最新入職的員工

SELECT

FROM

emp AS t1

INNER JOIN (

SELECT

post,

max(hire_date) max_date

FROM

emp

GROUP BY

post

) AS t2 ON t1.post = t2.post

WHERE

t1.hire_date = t2.max_date;

查詢語句關(guān)鍵字執(zhí)行順序

一個完整的mysql的查詢語句如下：

SELECT DISTINCT

FROM

JOIN

WHERE

GROUP BY

HAVING

ORDER BY

LIMIT

其關(guān)鍵字執(zhí)行順序如下：

(7) SELECT

(8) DISTINCT

(1) FROM

(3) JOIN

(2) ON

(4) WHERE

(5) GROUP BY

(6) HAVING

(9) ORDER BY

(10) LIMIT

總結(jié)

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