day--39-MySQL的多表查詢
多表查詢
一：表的基本介紹
能夠參考：https://www.cnblogs.com/cdf-opensource-007/p/6517627.html
創建一個員工表信息表和一個部門表，每一個員工都對應在哪一個部門。所以這兩張表具備必定關係。須要將兩個表格進行查詢，找到一個
員工所對應的所屬部門。
01：創建一個員工表：employee 和一個部門表departmentemployee表：
mysql> 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 -> ); Query OK, 0 rows affected (0.18 sec) department表： mysql> create table department( -> id int, -> name varchar(20) -> ); Query OK, 0 rows affected (0.12 sec)
02：給兩張表加入信息。
employee表：
mysql> insert into employee(name,sex,age,dep_id) values -> ("egon","male",18,200), -> ("alex","female",48,201), -> ("wupeiqi","male",38,210), -> ("yuanhao","female",28,202), -> ("liwenzhou","female",28,202), -> ("jingwen","female",18,204); Query OK, 6 rows affected (0.05 sec) Records: 6 Duplicates: 0 Warnings: 0
查看employee的結構：html
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 | | +--------+-----------------------+------+-----+---------+----------------+ 5 rows in set (0.00 sec)
查看employee的信息：python
mysql> select * from employee; +----+-----------+--------+------+--------+ | id | name | sex | age | dep_id | +----+-----------+--------+------+--------+ | 1 | egon | male | 18 | 200 | | 2 | alex | female | 48 | 201 | | 3 | wupeiqi | male | 38 | 210 | | 4 | yuanhao | female | 28 | 202 | | 5 | liwenzhou | female | 28 | 202 | | 6 | jingwen | female | 18 | 204 | +----+-----------+--------+------+--------+ 6 rows in set (0.00 sec)
department表：
mysql> insert into department values -> (200,"技術"), -> (201,"人力資源"), -> (202,"銷售"), -> (203,"運營"); Query OK, 4 rows affected (0.04 sec) Records: 4 Duplicates: 0 Warnings: 0 department的結構： mysql> desc department; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.00 sec)
department的信息：
mysql> select * from department; +------+--------------+ | id | name | +------+--------------+ | 200 | 技術 | | 201 | 人力資源 | | 202 | 銷售 | | 203 | 運營 | +------+--------------+ 4 rows in set (0.00 sec)
二：多表查詢（以employee和department爲例）mysql
#重點：外連接語法： select 字段列表 from 表1 inner/left/right join 表2 on 表1.字段=表2.字段；
01:交叉連接：生成笛卡兒積：
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 | 210 | 200 | 技術 | | 3 | wupeiqi | male | 38 | 210 | 201 | 人力資源 | | 3 | wupeiqi | male | 38 | 210 | 202 | 銷售 | | 3 | wupeiqi | male | 38 | 210 | 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 | female | 28 | 202 | 200 | 技術 | | 5 | liwenzhou | female | 28 | 202 | 201 | 人力資源 | | 5 | liwenzhou | female | 28 | 202 | 202 | 銷售 | | 5 | liwenzhou | female | 28 | 202 | 203 | 運營 | | 6 | jingwen | female | 18 | 204 | 200 | 技術 | | 6 | jingwen | female | 18 | 204 | 201 | 人力資源 | | 6 | jingwen | female | 18 | 204 | 202 | 銷售 | | 6 | jingwen | female | 18 | 204 | 203 | 運營 | +----+-----------+--------+------+--------+------+--------------+ 24 rows in set (0.00 sec) 這種顯示是將全部列表排序都生成了，咱們要找的排序確定會在這個笛卡兒積表格中，可是不利於分析。
02：內連接，只連接匹配的行。
mysql> select * from employee,department where department.id=employee.dep_id; #where條件分析。 +----+-----------+--------+------+--------+------+--------------+ | 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 | | 4 | yuanhao | female | 28 | 202 | 202 | 銷售 | | 5 | liwenzhou | female | 28 | 202 | 202 | 銷售 | +----+-----------+--------+------+--------+------+--------------+ 4 rows in set (0.00 sec)
#再看一個需求，我要查出技術部的員工的名字
mysql> select name from employee,department where employee.dep_id=department.id and department.name='技術'; ERROR 1052 (23000): Column 'name' in field list is ambiguous #上面直接就報錯了，由於select後面直接寫的name，在兩個表合併起來的表中，是有兩個name字段的，直接寫name是不行的，要加上表名，再看： mysql> select employee.name from employee,department where employee.dep_id=department.id and department.name="技術"; +------+ | name | +------+ | egon | +------+ 1 row in set (0.00 sec)
03:外連接之左連接：優先顯示左邊表的所有記錄sql
#以左表爲準，即找出全部員工信息，固然包括沒有部門的員工 #本質就是：在內連接的基礎上增長左邊有而右邊沒有的結果 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 | | 2 | alex | 人力資源 | | 4 | yuanhao | 銷售 | | 5 | liwenzhou | 銷售 | | 3 | wupeiqi | NULL | | 6 | jingwen | NULL | +----+-----------+--------------+ 6 rows in set (0.00 sec)
04:外連接之右連接：優先顯示右邊表的所有記錄
#以右表爲準，即找出全部員工信息，固然包括沒有部門的員工 #本質就是：在內連接的基礎上增長右邊有而左邊沒有的結果 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 | | 4 | yuanhao | 銷售 | | 5 | liwenzhou | 銷售 | | NULL | NULL | 運營 | +------+-----------+--------------+ 5 rows in set (0.00 sec)
05:全外連接：顯示左右兩個表的所有內容
001：union顯示的左右兩邊的數據，並將重複的數據去重。
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 | sex | age | dep_id | id | name | +------+-----------+--------+------+--------+------+--------------+ | 1 | egon | male | 18 | 200 | 200 | 技術 | | 2 | alex | female | 48 | 201 | 201 | 人力資源 | | 4 | yuanhao | female | 28 | 202 | 202 | 銷售 | | 5 | liwenzhou | female | 28 | 202 | 202 | 銷售 | | 3 | wupeiqi | male | 38 | 210 | NULL | NULL | | 6 | jingwen | female | 18 | 204 | NULL | NULL | | NULL | NULL | NULL | NULL | NULL | 203 | 運營 | +------+-----------+--------+------+--------+------+--------------+ 7 rows in set (0.06 sec)
002：union all 顯示的是左右兩邊表的數據，並不會去重
mysql> select * from employee left join department on employee.dep_id=department.id -> union all -> select * from employee right join department on employee.dep_id=department.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 | | 4 | yuanhao | female | 28 | 202 | 202 | 銷售 | | 5 | liwenzhou | female | 28 | 202 | 202 | 銷售 | | 3 | wupeiqi | male | 38 | 210 | NULL | NULL | | 6 | jingwen | female | 18 | 204 | NULL | NULL | | 1 | egon | male | 18 | 200 | 200 | 技術 | | 2 | alex | female | 48 | 201 | 201 | 人力資源 | | 4 | yuanhao | female | 28 | 202 | 202 | 銷售 | | 5 | liwenzhou | female | 28 | 202 | 202 | 銷售 | | NULL | NULL | NULL | NULL | NULL | 203 | 運營 | +------+-----------+--------+------+--------+------+--------------+ 11 rows in set (0.00 sec)
三：符合條件接連的查詢
一、
#示例1：之內鏈接的方式查詢employee和department表，而且employee表中的age字段值必須大於25,即找出年齡大於25歲的員工以及員工所在的部門 mysql> select employee.name,department.name from employee inner join department -> on employee.dep_id=department.id -> where age>25; +-----------+--------------+ | name | name | +-----------+--------------+ | alex | 人力資源 | | yuanhao | 銷售 | | liwenzhou | 銷售 | +-----------+--------------+ 3 rows in set (0.00 sec)
二、#實例：之內鏈接的方式查詢employee和department表，而且以age字段的生序方式顯示；
實例01：內鏈接的方式查詢employee和department表，而且employee表中的age字段值必須大於25,即找出年齡大於25歲的員工以及員工所在的部門 mysql> select employee.name,department.name from employee inner join department -> on employee.dep_id=department.id where age>25 -> order by age; #(order by 排序是默認的升序的，升序order by asc) +-----------+--------------+ | name | name | +-----------+--------------+ | yuanhao | 銷售 | | liwenzhou | 銷售 | | alex | 人力資源 | +-----------+--------------+ 3 rows in set (0.00 sec)
實例02：之內鏈接的方式查詢employee和department表，而且以age字段的升序方式顯示 mysql> select employee.id,employee.name,employee.age,department.name from employee,department -> where employee.dep_id=department.id -> and age >25 -> order by age asc; +----+-----------+------+--------------+ | id | name | age | name | +----+-----------+------+--------------+ | 5 | liwenzhou | 28 | 銷售 | | 4 | yuanhao | 28 | 銷售 | | 2 | alex | 48 | 人力資源 | +----+-----------+------+--------------+ 3 rows in set (0.00 sec)
四：子查詢
解釋：子查詢就是將一個查詢結果用括號括起來，交給另一個sql語句，做爲它的一個查詢語句來進行操做。 子查詢： #1：子查詢是將一個查詢語句嵌套在另外一個查詢語句中。 #2：內層查詢語句的查詢結果，能夠爲外層查詢語句提供查詢條件。 #3：子查詢中能夠包含：IN、NOT IN、ANY、ALL、EXISTS 和 NOT EXISTS等關鍵字 #4：還能夠包含比較運算符：= 、 !=、> 、<等
1: 帶in關鍵字的的字查詢：
mysql> select id,name from department where id in (select dep_id from employee group by -> dep_id having avg(age)>25); +------+--------------+ | id | name | +------+--------------+ | 201 | 人力資源 | | 202 | 銷售 | +------+--------------+ 2 rows in set (0.05 sec) 總結：子查詢的思路和解決問題同樣，先解決一個而後拿着這個的結果再去解決另一個問題，連表的思路是先將兩個表關聯在一塊兒， 而後在進行group by啊過濾啊等等操做，二者的思路是不同的.
01:#查詢員工平均年齡在25歲以上的部門名，能夠用連表，也能夠用子查詢， mysql> select department.name from department inner join employee on department.id=employee.dep_id -> group by department.name having avg(age)>25; +--------------+ | name | +--------------+ | 人力資源 | | 銷售 | +--------------+
02:#查看技術部員工姓名 mysql> select name from employee where dep_id in (select id from department where name="技術"); +------+ | name | +------+ | egon | +------+ 1 row in set (0.00 sec)
03:#查看不足1人的部門名(子查詢獲得的是有人的部門id) mysql> select id,name from department where id not in (select distinct dep_id from employee); +------+--------+ | id | name | +------+--------+ | 203 | 運營 | +------+--------+ 1 row in set (0.00 sec)
2:比較運算符的子查詢：
#比較運算符：=、!=、>、>=、<、<=、<> #查詢大於全部人平均年齡的員工名與年齡 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)
3：帶exists(存在）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；當返回值爲False時，外層查詢語句不進行查詢。還能夠寫not exists，和exists的效果就是反的
01：employee表中不存在id爲200
mysql> select * from employee where exists (select id from employee where id=500); Empty set (0.00 sec)
02：employee存在的狀況：
#只有當括號裏面的條件成立了，select * from employee 纔會執行 mysql> select * from employee where exists (select dep_id from employee where dep_id=200); +----+-----------+--------+------+--------+ | id | name | sex | age | dep_id | +----+-----------+--------+------+--------+ | 1 | egon | male | 18 | 200 | | 2 | alex | female | 48 | 201 | | 3 | wupeiqi | male | 38 | 210 | | 4 | yuanhao | female | 28 | 202 | | 5 | liwenzhou | female | 28 | 202 | | 6 | jingwen | female | 18 | 204 | +----+-----------+--------+------+--------+ 6 rows in set (0.00 sec)
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