含義：又稱為多表查詢，當(dāng)查詢的字段來(lái)自多個(gè)表時(shí)，就會(huì)用到連接查詢

笛卡爾乘積現(xiàn)象

表1 有m行，表2有n行，結(jié)果為m*n行

發(fā)生原因：沒(méi)有有效的連接條件。

如何避免：添加有效的連接條件。

案例：在boys表中匹配beauty表中女生的男朋友

以下的方法錯(cuò)誤，會(huì)出現(xiàn)笛卡爾乘積現(xiàn)象。

正確解法：

SELECT

NAME,

boyName

FROM

boys,

beauty

WHERE beauty.`boyfriend_id` = boys.`id` ;

結(jié)果：

6.1 連接查詢的分類

按年代分類：SQL192標(biāo)準(zhǔn)：僅僅支持內(nèi)連接

SQL199標(biāo)準(zhǔn)【推薦】：支持內(nèi)連接+外連接(左外和右外)+交叉連接

按功能分類：

1.內(nèi)連接：等值連接

非等值連接

自連接

2.外連接：左外連接

右外連接

全外連接

交叉連接

6.2 SQL192連接

1、等值連接

語(yǔ)法：①多表等值連接的結(jié)果為多表的交集部分

②n表連接，至少需要n-1個(gè)連接條件

③多表的順序沒(méi)有要求

④一般需要為表起別名

⑤可以搭配前面介紹的所有子句使用，比如排序、分組、篩選

案例1：查詢女神和對(duì)應(yīng)的男神名

SELECT

NAME,

boyName

FROM

boys,

beauty

WHERE beauty.`boyfriend_id` = boys.`id` ;

案例2：查詢部門名和對(duì)應(yīng)的部門名

SELECT

last_name,

department_name

FROM

employees,

departments

WHERE employees.`department_id` = `departments`.`department_id` ;

(1) 為表起別名提高語(yǔ)句的簡(jiǎn)潔度

區(qū)分多個(gè)重名的字段

注意：如果為表起了別名，則查詢的字段就不能使用原來(lái)的表名去限定

案例：查詢員工名、工種號(hào)、工種名

SELECT

e.last_name,

e.job_id,

j.job_title

FROM

employees e,

jobs j

WHERE e.`job_id` = j.`job_id`;

兩個(gè)表的順序可以調(diào)換：

下面代碼和上面功能一樣

SELECT

e.last_name,

e.job_id,

j.job_title

FROM

jobs j,

employees e

WHERE e.`job_id` = j.`job_id`;

(2)可以加篩選

案例1：查詢有獎(jiǎng)金的員工名、部門名

SELECT

last_name,

department_name

FROM

employees e,

departments d

WHERE e.`department_id` = d.`department_id`

AND e.`commission_pct` IS NOT NULL ;

案例2：查詢城市名中第二個(gè)字符為o的部門

SELECT

department_name,

city

FROM

departments d,

locations l

WHERE d.`location_id` = l.`location_id`

AND city LIKE '_o%' ;

結(jié)果：

(3)可以加分組

案例1：查詢每個(gè)城市的部門個(gè)數(shù)

SELECT

COUNT(*) AS "個(gè)數(shù)",

city

FROM

departments d,

locations l

WHERE d.`location_id` = l.`location_id`

GROUP BY city ;

案例2：查詢有獎(jiǎng)金的每個(gè)部門的部門名和部門的領(lǐng)導(dǎo)編號(hào)和該部門的最低工資

SELECT

department_name,

d.manager_id,

MIN(salary)

FROM

departments d,

employees e

WHERE d.`department_id` = e.`department_id`

AND commission_pct IS NOT NULL

GROUP BY department_name,d.`manager_id` ;

(4)可以加排序

案例：查詢每個(gè)工種的工種名和員工的個(gè)數(shù)

SELECT

job_title,

COUNT(*)

FROM

employees e,

jobs j

WHERE e.`job_id` = j.`job_id`

GROUP BY job_title

ORDER BY COUNT(*) DESC ;

(5)可以實(shí)現(xiàn)三表連接

案例：查詢員工名、部門名和所在的城市

SELECT

last_name,

department_name,

city

FROM

employees e,

departments d,

locations l

WHERE e.`department_id` = d.`department_id`

AND d.`location_id` = l.`location_id` ;

2、非等值連接

語(yǔ)法：

案例1：查詢員工的工資和工資級(jí)別

SELECT

salary,

grade_level

FROM

employees e,

job_grades g

WHERE salary BETWEEN g.`lowest_sal` AND g.`highest_sal` ;

工作原理：

用employees表中的salary去和job_grades對(duì)比，比如查詢24000的所在的等級(jí)，先用24000和1000及2999對(duì)比，如果介于這之間，則為等級(jí)A，依次分析，最終得到24000的等級(jí)為E。

3、自連接

語(yǔ)法：

案例：查詢員工名和上級(jí)的名稱

同一張表查兩次：比如查找Neena的上級(jí)是誰(shuí)。

對(duì)上面分析的進(jìn)一步拆分：

兩次查詢相當(dāng)于把同一張表分別看著員工表和領(lǐng)導(dǎo)表，然后通過(guò)manager_id等于employee_id來(lái)查詢。

本題解答：

SELECT

e.employee_id,

e.last_name AS "員工",

m.employee_id,

m.last_name AS "上級(jí)"

FROM

employees e,

employees m

WHERE e.`manager_id` = m.`employee_id` ;

小測(cè)試

1.顯示員工的最大工資，工資平均值

SELECT MAX(salary),AVG(salary) FROM employees;

2.查詢員工表的employees_id,job_id,last_name,按department_id降序，salary升序

SELECT

employee_id,

job_id,

last_name

FROM

employees

ORDER BY department_id DESC,salary ASC ;

3.查詢員工表的job_id中包含a和e的，并且a在e的前面

SELECT

job_id

FROM

employees

WHERE job_id LIKE '%a%e%' ;

4.

SELECT

s.name,

g.name,

r.score

FROM

student s,

grade g,

result r

WHERE s.id = r.studentNo AND g.id = s.gradeid ;

5.顯示當(dāng)前日期，以及去前后空格，截取子字符串的函數(shù)

6.顯示所有員工的姓名，部門號(hào)和部門名稱

SELECT

last_name,

d.department_id,

department_name

FROM

employees e,

departments d

WHERE e.`department_id` = d.`department_id` ;

7.查詢90號(hào)部門員工的job_id和90號(hào)部門的location_id

SELECT

job_id,

location_id

FROM

employees e,

departments d

WHERE e.`department_id` = d.`department_id` AND e.`department_id` = 90 ;

8.選擇所有有獎(jiǎng)金的員工的last_name ,department_id ,location_id ,city

SELECT

last_name,

department_name,

l.location_id,

city

FROM

employees e,

departments d,

locations l

WHERE e.`department_id` = d.`department_id`

AND d.`location_id` = l.`location_id`

AND e.`commission_pct` IS NOT NULL ;

9.選擇city在Toronto工作的員工的last_name ,job_id ,department_id ,department_name

SELECT

last_name,

job_id,

d.department_id,

department_name

FROM

employees e,

departments d,

locations l

WHERE e.`department_id` = d.`department_id`

AND d.`location_id` = l.`location_id`

AND city = 'Toronto' ;

10.查詢每個(gè)工種、每個(gè)部門的部門號(hào)、工種號(hào)和最低工資

SELECT

department_name,

job_title,

MIN(salary) AS "最低工資"

FROM

employees e,

departments d,

jobs j

WHERE e.`department_id` = d.`department_id`

AND j.`job_id` = e.`job_id`

GROUP BY department_name,job_title ;

11.查詢每個(gè)國(guó)家下的部門個(gè)數(shù)大于2的國(guó)家編號(hào)

SELECT

country_id,

COUNT(*) 部門個(gè)數(shù)

FROM

departments d,

locations l

WHERE d.`location_id` = l.`location_id`

GROUP BY country_id

HAVING COUNT(*) > 2 ;

12.選擇指定員工的姓名，員工號(hào)，以及他的管理者的姓名和員工號(hào)，結(jié)果類似于下面的格式

SELECT

e.`last_name` employees,

e.`employee_id` "Emp#",

m.`last_name`,

m.`employee_id` = "Mgr#"

FROM

employees e,

employees m

WHERE e.`manager_id` = m.`employee_id`

AND e.`last_name` = "kochhar" ;

總結(jié)

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