MySQL中有哪些資料查詢語句

一、基本概念（查詢語句）

①基本語句

1、「select * from 表名; ”，—可查詢表中全部資料；
2、“select 欄位名稱from 表名;”，—可查詢表中指定欄位的資料；
3、“select distinct 欄位名稱from 表名;”，—可對錶中資料進行去重查詢。
4、“select 欄位名稱from 表名where 查詢條件;”，—可根據條件查詢表中指定欄位的資料；

②條件查詢

#1）比較運算子：>, =,

查詢大於18歲的資訊

select * from students where age>18;
select id, name,gender from students where age>18;

查詢小於18歲的資訊

select * from students where age<18;

查詢年齡為18歲的所有學生的名字

select * from students where age=18;

2）邏輯運算子：and,或, not

–18到28之間的學生資訊

select * from students where age>18_and age<28:

–18歲以上的女性

select * from students where age>18 and gender="女";
select * from students where age>18 and gender=2;

#–18以上或身高查過180(包含)以上

select * from students where age>18 or height>=180;

不在18歲以上的女性這個範圍內的信息

select * from students where not (age>18 and gender=2);

年齡不是小於或等於18並且是女性

select * from students where (not age<=18) and gender=2;

3)模糊查詢：like， rlike

% 替換1個或多個
_ 替換1個
查詢姓名中以“小”開始的名字

select name from students where name="小";
select name from students where name like"小%";

查詢姓名中有「小」所有的名字

select name from students whece name like "%小%";

#查詢有2個字的名字

select name from students where name like "__";

查詢有3個字的名字

select name from students where name like "__";

查詢至少有2個字的名字select name from

students where name like "__%";

rlike正則
查詢以周開始的姓名

select name from students where name rlike "^周.*";

查詢以周開始、倫結尾的姓名

select name from students where name rlike "^周.*伦$";

4)範圍查詢：in，not in，between…and，not between…and

查詢年齡為18、34的姓名

select name, age from students where age=18 or age=34;
select name,age from students where age in (18，34);

not in不非連續的範圍之內
年齡不是18、34歲之間的信息

select name,age from students where age not in (18，34);

between … and …表示在一個連續的範圍內
查詢年齡在18到34之間的信息

select name,age from students where age between 18 and 34;

not between … and …表示不在一個連續的範圍內
查詢年齡不在在18到34之間的信息

select * from students where age not between 18 and 34;

空判斷

判空is null
查詢身高為空的資訊

select *from students where height is null/NULL/Null;

判非空is not null

select * from students where height is not null;

排序：order_by

–查詢年齡在18到34歲之間的男性，依照年齡從小到大排序

select * from students where (age between 18 and 34) and gender=1; 
select * from students where (age between 18 and 34) and gender=1 order by age; 
select * from students where (age between 18 and 34) and gender=1 order by age asc;

查詢年齡在18到34歲之間的女性，身高從高到矮排序

select * from students where (age between18 and 34) and gender=2 order by height desc;

order by多個字段
查詢年齡在18到34歲之間的女性，身高從高到矮排序，如果身高相同的情況下按照年齡從小到大排序

select * from students where (age between 18 and 34) and gender=2 order by height desc,age asc;

查詢年齡在18到34歲之間的女性，身高從高到矮排序，如果身高相同的情況下依照年齡從小到大排序，如果年齡也相同那麼依照id從大到小排序

select * from students where (age between 18 and 34) and gender=2 order by height desc,age asc, id desc;

依照年齡從小到大、身高從高到矮的排序

select * from students order by age asc,height desc;

分組：group_by, group_concat():查詢內容， having

where :是對整個資料表資訊的判斷;
having:是對於分組後的資料進行判斷
–group by
依照性別分組,查詢所有的性別

select gender from students group by gender;

–計算每個性別中的人數

select gender, count(*) from students group by gender;

where是在group by前面
–計算男性的人數

select count(*) from students where gender='男';

–group_concat(…)
查詢同種性別中的姓名

select gender,group_concat(name) from students group by gender;

having :having是在group by後面
查詢平均年齡超過30歲的性別，以及姓名

select gender ,avg(age) from students group by gender having avg(age) > 30;

查詢每種性別中的人數多於2個的信息

select gender,count(*) from students group by gender having count(*) > 2;

– 查詢每組性別的平均年齡

select gender,avg(age) from students group by gender;

分頁：limit

limit start,count   (start:表示从哪─个开始;count:表示数量)
即limit（第N页-1)*每个的个数，每页的个数;
limit在使用的时候，要放在最后面.

限制查詢出來的資料數量

select *from students where gender=1 limit 2;

查詢前5個資料

select* from students limit 0,5;

查詢id6-10(包含)的書序

select * from students limit 5,5;

每頁顯示2個，第1個頁

select * from students limit 0,2;

每頁顯示2個，第2個頁

select * from students limit 2,2;

每頁顯示2個，第3個頁面

select * from students limit 4,2;

每頁顯示2個，第4個頁面

select * from students limit 6,2;

每頁顯示2個，顯示第6頁的資訊,依照年齡從小到大排序

select * from students order by age asc limit 10,2;

– 如果重新排序了,那麼會顯示第一頁

select * from students where gender=2 order by height des limit 0,2;

5)聚合函數：count(), max() , min(), sum(), avg(), round()

聚合函數
-總數-- count
-查詢男性有多少人，女性有多少人

select count(*) from students where gender=1;
select count(*) as 男性人数 from students where gender=1;
select count(*) as 女性人数 from students where gender=2;

-最大值-最小值
– max --min
一查詢最大的年齡

select max (age) from students;

–查詢女性的最高身高

select max (height) from students where gender=2;

-求和
–sum
-計算所有人的年齡總和

select sum ( age) from students;

–平均值
– avg
–計算平均年齡

select avg(age) from students;

–計算平均年齡

select sum ( age) / count(* ) from students;

–四舍五入round ( 123.23 ,_1)保留1位小数
–计算所有人的平均年龄,保留2位小数

select round (sum(age)/count(*)，2) from students; select round ( sum(age)/count(*)，3) from students ;

–计算男性的平均身高保留2位小数

select round(avg (height),2) from students where gender=1; select name,round(avg(height)，2) from students where gender=1;

6)连接查询 ：inner join, left join, right join

inner join
select … from 表 A inner join表B;

select * from students inner join classes;

查询有能够对应班级的学生以及班级信息

select * from students inner join classes on students.cls_id=classes.id;

按照要求显示姓名、班级

select students.*, classes.name from students inner join classes on students.cls_id=classes.id;
select students.name，classes.name from students inner join classes on students.cls_id=classes.id;

给数据表起名字

select s.name,c.name from students as s inner join classes as c on s.cls_id=c.id;

查询有能够对应班级的学生以及班级信息，显示学生的所有信息，只显示班级名称

select s.*,c.name from students as s inner join classes as c on s.cls_id=c.id;

在以上的查询中,将班级姓名显示在第1列

select c.name，s.* from students as s inner join classes as c on s.cls_id=c.id;

查询有能够对应班级的学生以及班级信息，按照班级进行排序
select c.xxx s.xxx from student as s inner join clssses as c on … order by …;

select c.name，s.* from students as s inner join classes as c on s.cls_id=c.id order by c.name;

当时同一个班级的时候，按照学生的id进行从小到大排序

select c.name，s.* from students as s inner join classes as c on s.cls_id=c.id order by c.name,s.id;

left join
查询每位学生对应的班级信息

select * from students as s left join classes as c on s.cls_id=c.id;

查询没有对应班级信息的学生
– select … from xxx as s left join xxx as c on… where …
– select … from xxx as s left join xxx as c on… . … having …

select * from students as s left join classes as c on s.cls_id=c.id having c.id is null;
select * from students as s left join classes as c on s.cls_id=c.id where c.id is null;

left join是按照左边的表为基准和右边的表进行查询，查到就显示，查不到就显示为null

补充

查询所有字段：select * from 表名; 
查询指定字段：select 列1,列2,... from 表名; 
使用 as 给字段起别名： select 字段 as 名字.... from 表名; 
查询某个表的某个字段：select 表名.字段 .... from表名； 
可以通过 as 给表起别名： select 别名.字段 .... from 表名 as 别名; 
消除重复行： distinct 字段

注意：WHERE子句中是不能用聚集函数作为条件表达式的！

二、总结

1、普通查询

（1）命令：select * from ;

（2）命令：select from ;

2、去重查询（distinct）

命令：select distinct from

3、排序查询（order by）

升序：asc

降序：desc

降序排列命令：select from order by desc

不加desc一般默认为升序排列

4、分组查询(group by)

命令：select , Sum(score) from group by

假设现在又有一个学生成绩表(result)。要求查询一个学生的总成绩。我们根据学号将他们分为了不同的组。

命令：

select id, Sum(score) 
from result 
group by id;

现在有两个表学生表（stu）和成绩表（result）。

5.等值查询

当连接运算符为“=”时，为等值连接查询。

现在要查询年龄小于20岁学生的不及格成绩。

select stu.id,score 
from stu,result 
where stu.id = result.id and age < 20 and score < 60;

等值查询效率太低

6.外连接查询

①语法

select f1,f2,f3,....
from table1 left/right outer join table2
on 条件;

②左外连接查询，例如

select a.id,score
from
  (select id,age from stu where age < 20) a (过滤左表信息)
left join
  (select id, score from result where score < 60) b (过滤右表信息)
on a.id = b.id;

左外连接就是左表过滤的结果必须全部存在。右表中如果没有与左表过滤出来的数据匹配的记录，那么就会出现NULL值

③右外连接查询，例如

select a.id,score
from
  (select id,age from stu where age < 20) a (过滤左表信息)
right join
  (select id, score from result where score < 60) b (过滤右表信息)
on a.id = b.id;

右外连接就是左表过滤的结果必须全部存在

7.内连接查询

①语法

select f1,f2,f3,....
from table1 inter join table2
on 条件;

②例如

select a.id,score
from
  (select id,age from stu where age < 20) a (过滤左表信息)
inner join
  (select id, score from result where score < 60) b (过滤右表信息)
on a.id = b.id;

8.合并查询

在图书表(t_book)和图书类别表(t_bookType)中

①.union

使用union关键字是，数据库系统会将所有的查询结果合并到一起，然后去掉相同的记录；

select id 
from t_book  
 union 
select id 
from t_bookType;

②.union all

使用union all，不会去除掉重复的记录；

select id 
from t_book  
 union all 
select id 
from t_bookType;

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