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MySql——CRUD
2022-07-05 23:59:00 【lion tow】
目錄
一、基礎查詢
1、語法：select 查詢列錶 from 錶名
2、特點：
①、查詢列錶可以是：錶中的字段、常量值、錶達式、函數
②、查詢的結果是一個虛擬的錶格
3、 查詢語句：
3.1查詢錶中的單個字段：
SELECT last_name FROM t_mysql_employees;
3.2：查詢錶中的多個字段：
SELECT last_name,salary,email FROM t_mysql_employees; 3.3：查詢錶中的所有字段：(企業多用方法一，因為效率更快)
方式一：
SELECT
`employee_id`,
`first_name`,
`last_name`,
`phone_number`,
`last_name`,
`job_id`,
`phone_number`,
`job_id`,
`salary`,
`commission_pct`,
`manager_id`,
`department_id`,
`hiredate`
FROM
t_mysql_employees ;方式二：
SELECT * FROM t_mysql_employees;3.4：查詢常量值：
SELECT 100;
SELECT 'john';結果：
3.5：查詢常量值：
SELECT 100%98;結果：
3.6：查詢函數：
SELECT VERSION();結果：
3.7：起別名目的：
①便於理解
②如果要查詢的字段有重名的情况，使用別名可以區分開來
3.7.1：方式一：使用as：
SELECT 100%98 AS 結果;
SELECT salary AS "out put" FROM t_mysql_employees;運行結果：
3.7.2：方式二：使用空格：
SELECT last_name 姓,first_name 名 FROM t_mysql_employees;3.8：去重：把錶格中重複的字段去掉
SELECT DISTINCT department_id FROM t_mysql_employees; 3.9：+號的作用:與JAVA中的+作對比
java中的+號：
①運算符，兩個操作數都為數值型
②連接符，只要有一個操作數為字符串
mysql中的+號：只有唯一的功能：運算符
3.9.1+號的幾種情况：
1、兩個操作數都為數值型，則做加法運算：
select 100+90;運行結果：
2、只要其中一方為字符型，將字符型數值轉換成數值型，如果轉換成功，則繼續做加法運算：
select '123'+90結果：
3、如果轉換失敗，則將字符型數值轉換成0:
select 'john'+90;結果：
4、只要其中一方為null，則結果肯定為null:
select null+10;結果：
二、過濾和排序數據
1、按條件錶達式篩選：
SELECT * FROM t_mysql_employees WHERE salary>12000;2、按邏輯錶達式篩選：
案例1：查詢工資z在10000到20000之間的員工名、工資以及獎金
SELECT last_name, salary, commission_pct FROM t_mysql_employees WHERE salary>=10000 AND salary<=20000;3、模糊查詢：
3.1、like：
案例2：查詢員工名中包含字符a的員工信息
select * from employees where last_name like '%a%';3.2、between and：
案例3：查詢員工編號在100到120之間的員工信息
SELECT * FROM t_mysql_employees WHERE employee_id <= 120 AND employee_id>=100;對比一下：
SELECT
*
FROM
t_mysql_employees
WHERE
employee_id BETWEEN 100 AND 120;3.3、in：含義：判斷某字段的值是否屬於in列錶中的某一項
案例4：查詢員工的工種編號是 IT_PROG、AD_VP、AD_PRES中的一個員工名和工種編號
SELECT
last_name,
job_id
FROM
t_mysql_employees
WHERE
job_id = 'IT_PROT' OR job_id = 'AD_VP' OR JOB_ID ='AD_PRES'對比一下：
SELECT
last_name,
job_id
FROM
t_mysql_employees
WHERE
job_id IN( 'IT_PROT' ,'AD_VP','AD_PRES');3.4、is null is not null:
案例5：查詢沒有獎金的員工名和獎金率； 查詢有將近的員工名和獎金率
SELECT
last_name,
commission_pct
FROM
t_mysql_employees
WHERE
commission_pct IS NULL;
SELECT
last_name,
commission_pct
FROM
t_mysql_employees
WHERE
commission_pct IS NOT NULL;3.6安全等於 <=>
案例6：查詢沒有獎金的員工名和獎金率
SELECT
last_name,
commission_pct
FROM
t_mysql_employees
WHERE
commission_pct <=>NULL;is null 和安全等於區別：
IS NULL:僅僅可以判斷NULL值，可讀性較高，建議使用
<=> :既可以判斷NULL值，又可以判斷普通的數值，可讀性較低
4、order by 子句：排序查詢
語法：select 查詢列錶 from 錶名【where 篩選條件】order by 排序的字段或錶達式;
特點：
1、asc代錶的是昇序，可以省略desc代錶的是降序
2、order by子句可以支持 單個字段、別名、錶達式、函數、多個字段
3、order by子句在查詢語句的最後面，除了limit子句
4.1按單個字段排序：
SELECT * FROM t_mysql_employees ORDER BY salary DESC;4.2添加篩選條件再排序：
案例7：查詢部門編號>=90的員工信息，並按員工編號降序
SELECT * FROM t_mysql_employees WHERE department_id>=90 ORDER BY employee_id DESC;4.3按錶達式排序:
SELECT *,salary*12*(1+IFNULL(commission_pct,0)) FROM t_mysql_employees ORDER BY salary*12*(1+IFNULL(commission_pct,0)) DESC;4.4按錶達式排序:
案例8：查詢員工信息 按年薪昇序
SELECT *,salary*12*(1+IFNULL(commission_pct,0)) 年薪 FROM t_mysql_employees ORDER BY 年薪 ASC;4.5按函數排序:
案例9：查詢員工名，並且按名字的長度降序
SELECT LENGTH(last_name),last_name FROM t_mysql_employees ORDER BY LENGTH(last_name) DESC;4.6按函數排序:
案例10：查詢員工信息，要求先按工資降序，再按employee_id昇序
SELECT * FROM t_mysql_employees ORDER BY salary DESC,employee_id ASC;三、分組查詢
功能：用作統計使用，又稱為聚合函數或統計函數或組函數
分類： sum 求和、avg 平均值、max 最大值 、min 最小值 、count 計算個數
特點：1、sum、avg一般用於處理數值型 max、min、count可以處理任何類型
2、以上分組函數都忽略null值
3、可以和distinct搭配實現去重的運算
4、count函數的單獨介紹一般使用count(*)用作統計行數
5、和分組函數一同查詢的字段要求是group by後的字段
1、簡單的使用：
SELECT SUM(salary) FROM t_mysql_employees;
SELECT AVG(salary) FROM t_mysql_employees;
SELECT MIN(salary) FROM t_mysql_employees;
SELECT MAX(salary) FROM t_mysql_employees;
SELECT COUNT(salary) FROM t_mysql_employees;
2、參數支持哪些類型：
SELECT SUM(last_name) ,AVG(last_name) FROM t_mysql_employees;
SELECT SUM(hiredate) ,AVG(hiredate) FROM t_mysql_employees;
SELECT MAX(last_name),MIN(last_name) FROM t_mysql_employees;
SELECT MAX(hiredate),MIN(hiredate) FROM t_mysql_employees;
SELECT COUNT(commission_pct) FROM t_mysql_employees;
SELECT COUNT(last_name) FROM t_mysql_employees;
3、是否忽略null：
SELECT SUM(commission_pct) ,AVG(commission_pct),SUM(commission_pct)/35,SUM(commission_pct)/107 FROM t_mysql_employees;
SELECT MAX(commission_pct) ,MIN(commission_pct) FROM t_mysql_employees;
SELECT COUNT(commission_pct) FROM t_mysql_employees;
SELECT commission_pct FROM t_mysql_employees;4、和distinct搭配：
SELECT SUM(DISTINCT salary),SUM(salary) FROM t_mysql_employees;
SELECT COUNT(DISTINCT salary),COUNT(salary) FROM t_mysql_employees;5、count函數的詳細介紹：
SELECT COUNT(salary) FROM t_mysql_employees;
SELECT COUNT(*) FROM t_mysql_employees;
SELECT COUNT(1) FROM t_mysql_employees;6、分組函數一同查詢的字段有限制employee_id是最小的那個：
SELECT AVG(salary),employee_id FROM t_mysql_employees;最終案例分析：
一、錶結構要求：
-- 1.學生錶-t_student
-- sid 學生編號,sname 學生姓名,sage 學生年齡,ssex 學生性別
-- 2.教師錶-t_teacher
-- tid 教師編號,tname 教師名稱
-- 3.課程錶-t_course
-- cid 課程編號,cname 課程名稱,tid 教師名稱
-- 4.成績錶-t_score
-- sid 學生編號,cid 課程編號,score 成績
-- 學生錶
insert into t_student values('01' , '趙雷' , '1990-01-01' , '男');
insert into t_student values('02' , '錢電' , '1990-12-21' , '男');
insert into t_student values('03' , '孫風' , '1990-12-20' , '男');
insert into t_student values('04' , '李雲' , '1990-12-06' , '男');
insert into t_student values('05' , '周梅' , '1991-12-01' , '女');
insert into t_student values('06' , '吳蘭' , '1992-01-01' , '女');
insert into t_student values('07' , '鄭竹' , '1989-01-01' , '女');
insert into t_student values('09' , '張三' , '2017-12-20' , '女');
insert into t_student values('10' , '李四' , '2017-12-25' , '女');
insert into t_student values('11' , '李四' , '2012-06-06' , '女');
insert into t_student values('12' , '趙六' , '2013-06-13' , '女');
insert into t_student values('13' , '孫七' , '2014-06-01' , '女');
-- 教師錶
insert into t_teacher values('01' , '張三');
insert into t_teacher values('02' , '李四');
insert into t_teacher values('03' , '王五');
-- 課程錶
insert into t_course values('01' , '語文' , '02');
insert into t_course values('02' , '數學' , '01');
insert into t_course values('03' , '英語' , '03');
-- 成績錶
insert into t_score values('01' , '01' , 80);
insert into t_score values('01' , '02' , 90);
insert into t_score values('01' , '03' , 99);
insert into t_score values('02' , '01' , 70);
insert into t_score values('02' , '02' , 60);
insert into t_score values('02' , '03' , 80);
insert into t_score values('03' , '01' , 80);
insert into t_score values('03' , '02' , 80);
insert into t_score values('03' , '03' , 80);
insert into t_score values('04' , '01' , 50);
insert into t_score values('04' , '02' , 30);
insert into t_score values('04' , '03' , 20);
insert into t_score values('05' , '01' , 76);
insert into t_score values('05' , '02' , 87);
insert into t_score values('06' , '01' , 31);
insert into t_score values('06' , '03' , 34);
insert into t_score values('07' , '02' , 89);
insert into t_score values('07' , '03' , 98);
二、題目：
01）查詢" 01 "課程比" 02 "課程成績高的學生的信息及課程分數
select a.*,b.score,c.score from t_student a,t_score b,t_score c where a.sid=b.sid and a.sid=c.sid and b.cid='01' and c.cid='02' and b.score > c.score
02）查詢同時存在" 01 "課程和" 02 "課程的情况
select a.*,b.cid,c.cid from t_student a,t_score b,t_score c where a.sid=b.sid and a.sid=c.sid and b.cid='01' and c.cid='02'
03）查詢存在" 01 "課程但可能不存在" 02 "課程的情况(不存在時顯示為 null )
select a.*,b.cid,c.cid from t_student a,t_score b,t_score c where a.sid=b.sid and a.sid=c.sid and b.cid='01' and c.cid not in('02')
04）查詢不存在" 01 "課程但存在" 02 "課程的情况
select a.*,b.cid,c.cid from t_student a,t_score b,t_score c where a.sid=b.sid and a.sid=c.sid and c.cid not in('01') and b.cid='02'
05）查詢平均成績大於等於 60 分的同學的學生編號和學生姓名和平均成績
select a.sid,a.sname,AVG(b.score) from t_student a,t_score b where a.sid = b.sid GROUP BY b.sid HAVING AVG(b.score)>=60
06）查詢在t_score錶存在成績的學生信息
select * from t_student where sid not in(select DISTINCT(sid) from t_score)
07）查詢所有同學的學生編號、學生姓名、選課總數、所有課程的總成績(沒成績的顯示為 null )
select a.sid,a.sname,COUNT(b.cid),SUM(b.score) from t_student a,t_score b where a.sid = b.sid GROUP BY b.sid
08）查詢「李」姓老師的數量
select count(*) from t_teacher where tname like '李%'
09）查詢學過「張三」老師授課的同學的信息
select a.*,c.tname from t_student a , t_score b ,t_teacher c ,t_course d where a.sid=b.sid and b.cid = d.cid and d.tid = c.tid and b.cid='01'
10）查詢沒有學全所有課程的同學的信息
select * from t_student where sid not in(select a.sid from t_student a,t_score b,t_score c,t_score d where a.sid=b.sid and a.sid=c.sid and b.cid='01' and c.cid='02' and d.cid='03')
11）查詢沒學過"張三"老師講授的任一門課程的學生姓名
select sname from t_student where sname not in(select a.sname from t_student a , t_score b ,t_teacher c ,t_course d where a.sid=b.sid and b.cid = d.cid and d.tid = c.tid and b.cid='01')
12）查詢兩門及其以上不及格課程的同學的學號，姓名及其平均成績
select * from t_student a,t_score b where a.sid=b.sid and a.sid in(select c.sid from t_score c where c.score<60 GROUP BY c.sid HAVING COUNT(*)>1)
13）檢索" 01 "課程分數小於 60，按分數降序排列的學生信息
select a.* from t_student a,t_score b where a.sid=b.sid and b.cid='01' and b.score<60 ORDER BY b.score desc
14）按平均成績從高到低顯示所有學生的所有課程的成績以及平均成績
select a.*,AVG(b.score) from t_student a,t_score b where a.sid = b.sid GROUP BY b.sid HAVING AVG(b.score)>=0 ORDER BY AVG(b.score) desc
15）查詢各科成績最高分、最低分和平均分：
以如下形式顯示：課程 ID，課程 name，最高分，最低分，平均分，及格率，中等率，優良率，優秀率及格為>=60，中等為：70-80，優良為：80-90，優秀為：>=90
要求輸出課程號和選修人數，查詢結果按人數降序排列，若人數相同，按課程號昇序排列
select a.cid,cname,max(a.score)'最高分',min(a.score)'最低分',avg(a.score)'平均分',((select count(sid) from t_score where score>=60 and cid=b.cid )/(select count(sid) from t_score where cid=b.cid)) '及格率' from t_score a
inner join t_course b on a.cid = b.cid
group by b.cid;
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