MySQL DQL语句的具体使用

SELECT 字段1, 字段2, 字段3 ... FROM 表名 ;
SELECT * FROM 表名 ;

SELECT 字段1 [ AS 别名1 ] , 字段2 [ AS 别名2 ] ... FROM 表名;
SELECT 字段1 [ 别名1 ] , 字段2 [ 别名2 ] ... FROM 表名;

SELECT DISTINCT 字段列表 FROM 表名;

select name,workno,age from emp;

select id ,workno,name,gender,age,idcard,workaddress,entrydate from emp;

select workaddress as '工作地址' from emp;
-- as可以省略
select workaddress '工作地址' from emp;

select distinct workaddress '工作地址' from emp;

SELECT 字段列表 FROM 表名 WHERE 条件列表 ;

select * from emp where age = 22;

select * from emp where idcard is null;

select * from emp where age >= 15 && age <= 20;
select * from emp where age >= 15 and age <= 20;
select * from emp where age between 15 and 20;

select * from emp where age = 18 or age = 20 or age =40;
select * from emp where age in(18,20,40);

select * from emp where name like '__';

select * from emp where idcard like '%X';
select * from emp where idcard like '_________________X';

SELECT 聚合函数(字段列表) FROM 表名 ; 

select count(*) from emp; -- 统计的是总记录数
select count(idcard) from emp; -- 统计的是idcard字段不为null的记录数

select avg(age) from emp;

select max(age) from emp;

select min(age) from emp;

select sum(age) from emp where workaddress = '西安';