數據庫多表查詢
阿新 • • 發佈:2019-03-09
年齡 字段 sed group by clas pen 連接查詢 aps create
連表查詢
#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 ); #插入數據 insert into department values (200,‘技術‘), (201,‘人力資源‘), (202,‘數據準備銷售‘), (203,‘運營‘); insert into employee(name,sex,age,dep_id) values (‘egon‘,‘male‘,18,200), (‘alex‘,‘female‘,48,201), (‘wupeiqi‘,‘male‘,38,201), (‘yuanhao‘,‘female‘,28,202), (‘liwenzhou‘,‘male‘,18,200), (‘jingliyang‘,‘female‘,18,204) ; #查看表結構和數據 mysql> desc department; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ mysql> desc employee; +--------+-----------------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +--------+-----------------------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | name | varchar(20) | YES | | NULL | | | sex | enum(‘male‘,‘female‘) | NO | | male | | | age | int(11) | YES | | NULL | | | dep_id | int(11) | YES | | NULL | | +--------+-----------------------+------+-----+---------+----------------+ mysql> select * from department; +------+--------------+ | id | name | +------+--------------+ | 200 | 技術 | | 201 | 人力資源 | | 202 | 銷售 | | 203 | 運營 | +------+--------------+ mysql> select * from employee; +----+------------+--------+------+--------+ | 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 | +----+------------+--------+------+--------+
表的笛卡爾積
select * from 表1,表2
會直接將兩張表拼接在一起,不適用任何匹配條件,生成笛卡爾積
mysql> select * from employee,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 | +----+------------+--------+------+--------+------+--------------+ 24 rows in set (0.00 sec)笛卡爾積
內連接
inner join
select 字段 from 表1 inner join 表2 on 條件
只有兩張表中條件互相匹配的項才能被顯示出來
mysql> select * from employee as emp inner join department as dep on emp.dep_id = dep.id; +----+-----------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +----+-----------+--------+------+--------+------+--------------+ | 1 | egon | 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 | | 5 | liwenzhou | male | 18 | 200 | 200 | 技術 | +----+-----------+--------+------+--------+------+--------------+ 5 rows in set (0.00 sec) # 找出年齡大於25歲的員工及員工所在的部門 將內連接的表當做一個新的表,直接在後面寫where條件 mysql> select emp.name,dep.name from employee as emp inner join department as dep on emp.dep_id = dep.id where age >25; # name字段名重復 在select時要標明表名,age字段只有一個,條件中可以直接寫 +---------+--------------+ | name | name | +---------+--------------+ | alex | 人力資源 | | wupeiqi | 人力資源 | | yuanhao | 銷售 | +---------+--------------+ 3 rows in set (0.00 sec) # 查詢兩張表,並以age字段升序顯示 mysql> select * from employee as emp inner join department as dep on emp.dep_id = dep.id order by age; +----+-----------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +----+-----------+--------+------+--------+------+--------------+ | 1 | egon | male | 18 | 200 | 200 | 技術 | | 5 | liwenzhou | male | 18 | 200 | 200 | 技術 | | 4 | yuanhao | female | 28 | 202 | 202 | 銷售 | | 3 | wupeiqi | male | 38 | 201 | 201 | 人力資源 | | 2 | alex | female | 48 | 201 | 201 | 人力資源 | +----+-----------+--------+------+--------+------+--------------+ 5 rows in set (0.00 sec)內連接
外鏈接
左外連接
left join
select 字段 from 表1 left join 表2 on 條件
完整的顯示左表中的所有數據,根據條件顯示右表,沒有匹配的數據顯示null
mysql> select * from employee as emp left join department as dep on emp.dep_id = dep.id; +----+------------+--------+------+--------+------+--------------+ | 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 | +----+------------+--------+------+--------+------+--------------+ 6 rows in set (0.00 sec)左外連接
右外連接
right join
select 字段 from 表1 right join 表2 on 條件
完整的顯示右表中的所有數據,根據條件顯示左表,沒有匹配的數據顯示null
mysql> select * from employee as emp right join department as dep on emp.dep_id = dep.id; +------+-----------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +------+-----------+--------+------+--------+------+--------------+ | 1 | egon | 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 | | 5 | liwenzhou | male | 18 | 200 | 200 | 技術 | | NULL | NULL | NULL | NULL | NULL | 203 | 運營 | +------+-----------+--------+------+--------+------+--------------+ 6 rows in set (0.00 sec)
全外連接
union
select 字段 from 表1 left join 表2 on 條件
union
select 字段 from 表1 right join 表2 on 條件
將所有的內容都顯示出來,匹配不到的內容顯示null
mysql> select * from employee as emp left join department as dep on emp.dep_id = dep.id -> union -> select * from employee as emp right join department as dep on emp.dep_id = dep.id; +------+------------+--------+------+--------+------+--------------+ | 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 | +------+------------+--------+------+--------+------+--------------+ 7 rows in set (0.00 sec)
子查詢
將一個查詢語句嵌套在一個查詢語句中
效率比連表查詢低
# 查詢人數<1的部門名字 mysql> select name from department where id not in (select dep_id from employee group by dep_id); +--------+ | name | +--------+ | 運營 | +--------+ 1 row in set (0.00 sec) #查詢技術部門的員工名字 mysql> select name from employee where dep_id = (select id from department where name=‘技術‘); +-----------+ | name | +-----------+ | egon | | liwenzhou | +-----------+ 2 rows in set (0.00 sec) # 查詢大於所有員工平均年齡的名字和年齡 mysql> select name,age from employee where age > (select avg(age) from employee); +---------+------+ | name | age | +---------+------+ | alex | 48 | | wupeiqi | 38 | +---------+------+ 2 rows in set (0.00 sec)子查詢
連接查詢和子查詢一起使用
# 查詢大於部門內平均年齡的名字,年齡 mysql> select * from employee as t1 inner join (select dep_id,avg(age) as age from employee group by dep_id) as t2 on t1.dep_id = t2.dep_id where t1.age > t2.age; +----+------+--------+------+--------+--------+---------+ | id | name | sex | age | dep_id | dep_id | age | +----+------+--------+------+--------+--------+---------+ | 2 | alex | female | 48 | 201 | 201 | 43.0000 | +----+------+--------+------+--------+--------+---------+ 1 row in set (0.00 sec)示例
數據庫多表查詢