MySQL多表查詢詳解上

更新時間：2020年07月27日 17:30:17 作者：木卯生十木

這篇文章主要介紹了MySQL多表查詢詳解上，文中通過示例代碼介紹的非常詳細(xì)，對大家的學(xué)習(xí)或者工作具有一定的參考學(xué)習(xí)價值，需要的朋友們下面隨著小編來一起學(xué)習(xí)學(xué)習(xí)吧

時光在不經(jīng)意間，總是過得出奇的快。小暑已過，進(jìn)入中暑，太陽更加熱烈的綻放著ta的光芒，...在外面被太陽照顧的人們啊，你們都是勤勞與可愛的人啊。在房子里已各種姿勢看我這篇這章的你，既然點了進(jìn)來，那就由我繼續(xù)帶你回顧MySql的知識吧！

回顧練習(xí)資料girls庫以及兩張表的腳本：

鏈接: https://pan.baidu.com/s/1bgFrP7dBBwk3Ao755pU4Qg 提取碼: ihg7

引題:笛卡爾現(xiàn)象，先來觀看一下兩張表。

SELECT * FROM boys;

SELECT * FROM beauty;

SELECT NAME,boyname FROM boys,beauty;
最終結(jié)果:12*4=48行

#進(jìn)階6:連接查詢
含義:又稱多表查詢,當(dāng)查詢的字段來自于多個表時，就會用到連接查詢

笛卡兒積現(xiàn)象: 表1 有m行,表2有n行,結(jié)果=m*n行
產(chǎn)生原因:沒有有效的連接條件
解決方法:添加有效的連接條件
 連接分類:
   按年代分類:
   SQL1992標(biāo)準(zhǔn)(192標(biāo)準(zhǔn)):僅支持內(nèi)連接
   SQL1999標(biāo)準(zhǔn)(199標(biāo)準(zhǔn)) [推薦]:支持內(nèi)連接+外聯(lián)結(jié)(左外與右外)+交叉連接

  按功能分類:
   內(nèi)連接:
     等值連接
     非等值連接
     自聯(lián)結(jié)
   外連接:
     左外連接
     右外連接
     全外連接
    交叉連接:
      左外連接
      右外連接
      全外連接
   交叉連接:

SELECT NAME,boyname FROM boys,beauty
WHERE beauty.boyfriend_id = boys.id;

#一.SQL192標(biāo)準(zhǔn)
#1.等值連接
 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個連接條件
 ③多表的順序沒有要求
 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。
#多表查詢，先匹配在篩選
#案例1.查詢員工名和對應(yīng)的部門名。
SELECT first_name AS 名,department_name AS 部門名
FROM employees,departments
WHERE employees.department_id = departments.department_id;

#案例2.查詢員工名,工種號，工種名。對于兩張表共有的字段需要加表名作限定不然會報錯。
錯誤示例:
SELECT first_name AS 名,employees.job_id AS 工種號,job_title AS 工種名
FROM employees,jobs
WHERE employees.job_id = jobs.job_id;

#2.為表起別名
 ①提高語句的簡潔度
 ②區(qū)分多個重命名的字段
注意:如果為表起了別名,則查詢的字段就不能使用原來的表名去限定
SELECT first_name AS 名,e.job_id AS 工種號,job_title AS 工種名
FROM employees AS e,jobs AS j
WHERE e.job_id = j.job_id;

給表起了別名，再用表完整名子做限定會報錯，不允許。根據(jù)執(zhí)行順序走先走FROM，
走完FROM后就用別名的，相當(dāng)于生成了一個虛擬的視圖，不再認(rèn)原來的表名。

#3.兩個表名的順序是否可以調(diào)換，是可以調(diào)換的。
SELECT first_name AS 名,e.job_id AS 工種號,job_title AS 工種名
FROM jobs AS j,employees AS e
WHERE e.job_id = j.job_id;

#4.可以加篩選
#案例3.查詢有獎金的員工名,部門名。
SELECT first_name AS 名,department_name AS 部門名,commission_pct AS 獎金
FROM employees AS e,departments AS d
WHERE e.department_id=d.department_id
AND commission_pct IS NOT NULL;#AND e.commission_pct IS NOT NULL;

#案例4.查詢城市名中第二個字符為o對應(yīng)的城市名與部門名。
SELECT city AS 城市,department_name AS 部門名
FROM locations AS l,departments AS d
WHERE l.location_id = d.location_id
AND city LIKE '_o%';

#5.可以加分組
#案例1.查詢每個城市的部門個數(shù)。
SELECT city AS 城市,COUNT(department_id) AS 個數(shù)
FROM locations AS l,departments AS d
WHERE l.location_id = d.location_id
GROUP BY l.city;

#案例2.查詢有獎金的每個部門的部門名和部門的領(lǐng)導(dǎo)編號和該部門的最低工資。
#查詢的時候不確定把兩個列都加上。
SELECT commission_pct AS 獎金,department_name AS 部門名,
d.manager_id AS 領(lǐng)導(dǎo)編號,MIN(salary) AS 最低工資
FROM employees AS e,departments AS d
WHERE e.department_id = d.department_id
AND commission_pct IS NOT NULL
GROUP BY department_name,d.manager_id;

#6.可以加排序
#案例1:查詢每個工種的工種名和員工的個數(shù),并且按員工個數(shù)降序。
SELECT j.job_title AS 工種名,COUNT(employee_id) AS 個數(shù)
FROM employees AS e,jobs AS j
WHERE e.job_id = j.job_id
GROUP BY job_title
ORDER BY 個數(shù) DESC;

#7.三表連接
#案例1.查詢員工名,部門名與所在的城市
SELECT first_name AS 名,d.manager_id AS 部門名,city AS 城市
FROM employees AS e,departments AS d,locations AS l
WHERE e.department_id = d.department_id
 AND d.location_id = l.location_id;

#案例2.查詢員工名,部門名與所在的城市,城市以s開頭。
SELECT first_name AS 名,d.manager_id AS 部門名,city AS 城市
FROM employees AS e,departments AS d,locations AS l
WHERE e.department_id = d.department_id
AND d.location_id = l.location_id
 AND city LIKE 's%';

#案例3.查詢員工名,部門名與所在的城市,城市以s開頭,按姓名降序排列。
SELECT first_name AS 名,d.manager_id AS 部門名,city AS 城市
FROM employees AS e,departments AS d,locations AS l
WHERE e.department_id = d.department_id
AND d.location_id = l.location_id
AND city LIKE 's%'
ORDER BY department_name DESC;

新增一張工資等級表。
CREATE TABLE job_grades
(grade_level VARCHAR(3),
 lowest_sal int,
 highest_sal int);
INSERT INTO job_grades
VALUES ('A', 1000, 2999);
INSERT INTO job_grades
VALUES ('B', 3000, 5999);
INSERT INTO job_grades
VALUES('C', 6000, 9999);
INSERT INTO job_grades
VALUES('D', 10000, 14999);
INSERT INTO job_grades
VALUES('E', 15000, 24999);
INSERT INTO job_grades
VALUES('F', 25000, 40000);

#2.非等值連接,(范圍判斷)
#案例1.查詢員工的工資和工資級別。
SELECT salary AS 工資,grade_level AS 等級
FROM employees AS e,job_grades AS g
WHERE salary BETWEEN g.lowest_sal AND g.highest_sal;

#案例2.查詢員工的工資和工資級別,展示出A級別的員工。
SELECT salary AS 工資,grade_level AS 等級
FROM employees AS e,job_grades AS g
WHERE salary BETWEEN g.lowest_sal AND g.highest_sal
AND g.grade_level = 'A';

#3.自聯(lián)接[自己連接自己]

#案例1.查詢員工名和上級的名稱.
SELECT e.employee_id,e.last_name AS 員工,
m.employee_id,m.last_name AS 領(lǐng)導(dǎo)
FROM employees e,employees m
WHERE e.manager_id = m.employee_id;