SQL语句练习

本文章将给大家列举一下sql的语法。

sql的关键字有SELECT，FROM，GROUP BY，WHERE，ORDER BY，HAVING，DROP，ALTER，UNION，UNION ALL，LIKE

例子有四个表：学生表S(SID,SNAME,AGE,SEX)，课程表(CID,CNAME,TID)，成绩表(SID,CID,SCORE)，教师表(TID,TNAME)

建表的sql：https://blog.csdn.net/glgom/article/details/83062108

1.-- 统计各科成绩、各分数段人数（课程ID,课程名称，[100-85]优秀，[80-70]良，[70-60]中，[<60]差）

SELECT SC.CID ‘课程ID’,C.CNAME ‘课程名称’,
SUM(CASE WHEN SC.SCORE<=100 AND SC.SCORE>85 THEN 1 ELSE 0 END) AS ‘优秀’,
SUM(CASE WHEN SC.SCORE<=85 AND SC.SCORE>70 THEN 1 ELSE 0 END) AS ‘良’,
SUM(CASE WHEN SC.SCORE<=70 AND SC.SCORE>60 THEN 1 ELSE 0 END) AS ‘中’,
SUM(CASE WHEN SC.SCORE<=60 THEN 1 ELSE 0 END) AS ‘差’
FROM SC
LEFT JOIN C ON C.CID=SC.CID
GROUP BY SC.CID
或者
SELECT SC.CID ‘课程ID’,C.CNAME ‘课程名称’,
COUNT(CASE WHEN SC.SCORE<=100 AND SC.SCORE>85 THEN SCORE ELSE NULL END) AS ‘优秀’,
COUNT(CASE WHEN SC.SCORE<=85 AND SC.SCORE>70 THEN SCORE ELSE NULL END) AS ‘良’,
COUNT(CASE WHEN SC.SCORE<=70 AND SC.SCORE>60 THEN SCORE ELSE NULL END) AS ‘中’,
COUNT(CASE WHEN SC.SCORE<=60 THEN SCORE ELSE NULL END) AS ‘差’
FROM SC
LEFT JOIN C ON C.CID=SC.CID
GROUP BY SC.CID

2.-- 查询"001"课程比"002"课程成绩高的所有学生的学号

要考虑到课程的成绩可能是没有的，因为是001比002课程高，所以左连接002课程就好，如果为空的成绩就为0
SELECT T3.SID,T3.ONE,T3.TWO
FROM(
SELECT SC.SID,SC.SCORE ONE,CASE WHEN T1.SCORE THEN T1.SCORE ELSE 0 END TWO
FROM SC
LEFT JOIN SC T1 ON T1.SID = SC.SID AND T1.CID=‘002’
WHERE SC.CID=‘001’
)T3
WHERE T3.ONE>T3.TWO

3.-- 查询平均成绩大于60份的同学的学号和平均成绩

SELECT SID,AVG(SCORE) AVG_SCORE
FROM SC
GROUP BY SID
HAVING AVG(SCORE)>60

若是只想保留小数点的后两位，则
SELECT SID,CAST(AVG(SCORE) AS DECIMAL(18,2)) AVG
FROM SC
GROUP BY SID
HAVING AVG(SCORE)>60

– decimal(a,b),存储数据范围是：-10^38~1038-1 的固定精度和小数位的数字。a表示整个数值占据的位数，b为小数的位数，如decimal(10,2),数值一共有10位，整数占8位，小数占两位
– cast(expression AS data_type)是用来数据类型转换的，expression是任何有效的表达式

4.-- 查询没学过"凯尔"老师课的同学学号，姓名
SELECT S.SID,SNAME
FROM S
WHERE S.SID NOT IN(
SELECT DISTINCT SC.SID
FROM SC
WHERE SC.CID IN(
SELECT C.CID
FROM C,T
WHERE C.TID=T.TID AND T.TNAME=‘凯尔’
)
)
– distinct关键字，去重，必须放在开头，不能是select cid, distinct sid
;

5.-- 查询学过“语文”并且学过“物理”的学生信息
SELECT S.SID,S.SNAME,S.AGE,S.SEX
FROM S
INNER JOIN(
SELECT DISTINCT SC.SID
FROM SC
INNER JOIN C ON C.CID=SC.CID AND C.CNAME=‘语文’
)T1 ON T1.SID=S.SID
INNER JOIN(
SELECT DISTINCT SC.SID
FROM SC
INNER JOIN C ON C.CID=SC.CID AND C.CNAME=‘物理’
)T2 ON T2.SID=S.SID
;

6.-- 查询学过课程号’005’和课程号’033’的学生信息
SELECT S.SID,S.SNAME,S.AGE,S.SEX
FROM S,SC T1,SC T2
WHERE S.SID=T1.SID AND S.SID=T2.SID AND T1.CID=‘005’ AND T2.CID=‘033’
;

– 或者
SELECT S.SID,S.SNAME,S.AGE,S.SEX
FROM S
INNER JOIN(
SELECT DISTINCT SC.SID
FROM SC
WHERE SC.CID=‘005’
) T1 ON T1.SID=S.SID
INNER JOIN(
SELECT DISTINCT SC.SID
FROM SC
WHERE SC.CID=‘033’
) T2 ON T2.SID=S.SID

7.-- 查询学过’010’但没学过’002’的学生的信息
SELECT S.SID,S.SNAME,S.AGE,S.SEX
FROM S
WHERE S.SID IN(
SELECT DISTINCT SC.SID
FROM SC
WHERE SC.CID=‘010’
)
AND S.SID NOT IN(
SELECT DISTINCT SC.SID
FROM SC
WHERE SC.CID=‘002’
)
;

8.-- 查询每位同学的每门课程的成绩(学生学号，姓名，课程1，课程2，····)
SELECT S.SID,S.SNAME,
MAX(CASE SC.CID WHEN ‘001’ THEN SC.SCORE ELSE 0 END) ‘数据结构’,
MAX(CASE SC.CID WHEN ‘002’ THEN SC.SCORE ELSE 0 END) ‘语文’,
MAX(CASE SC.CID WHEN ‘005’ THEN SC.SCORE ELSE 0 END) ‘数学’,
MAX(CASE SC.CID WHEN ‘010’ THEN SC.SCORE ELSE 0 END) ‘物理’,
MAX(CASE SC.CID WHEN ‘011’ THEN SC.SCORE ELSE 0 END) ‘生物’,
MAX(CASE SC.CID WHEN ‘021’ THEN SC.SCORE ELSE 0 END) ‘化学’,
MAX(CASE SC.CID WHEN ‘033’ THEN SC.SCORE ELSE 0 END) ‘英语’
FROM S
LEFT JOIN SC ON SC.SID=S.SID
GROUP BY S.SID
– 用MAX是为了将无数据的点设为0，防止出现NULL
;

9.-- 查询老师"凯尔"和老师"克列"的学生信息
SELECT S.SID,S.SNAME,S.AGE,S.SEX
FROM S
INNER JOIN(
SELECT DISTINCT SC.SID
FROM SC
INNER JOIN C ON C.CID=SC.CID AND C.CID IN(
SELECT C.CID
FROM C
INNER JOIN T ON T.TID=C.TID AND T.TNAME=“凯尔”
)
)T1 ON T1.SID=S.SID
UNION
SELECT S.SID,S.SNAME,S.AGE,S.SEX
FROM S
INNER JOIN(
SELECT DISTINCT SC.SID
FROM SC
INNER JOIN C ON C.CID=SC.CID AND C.CID IN(
SELECT C.CID
FROM C
INNER JOIN T ON T.TID=C.TID AND T.TNAME=“克列”
)
)T1 ON T1.SID=S.SID
;
– UNION合并查询结果并去重，UNION ALL合并查询结果但不去重

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作者：glgom
来源：CSDN
原文：https://blog.csdn.net/glgom/article/details/82900890
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