MySQL - database query - basic query

The basic query

MySQL Database usage SELECT Statement to query data .

1. Query multiple fields

The following is in MySQL A common way to query data in a database SELECT grammar ：

SELECT  Field name , Field name ... FROM  Table name ;
SELECT * FROM  Table name ;                   # Query all fields 

• You can use one or more tables in a query statement , Use commas between tables (,) Division , And use WHERE Statement to set the query conditions .

• SELECT Command can read one or more records .

• You can use an asterisk （*） To replace other fields ,SELECT Statement will return all the field data of the table

2. Set alias

SELECT  Field 1 [AS  Alias ], Field 2 [AS  Alias ]... FROM  Table name ;

3. Remove duplicate records

SELECT DISTINCT  Field list  FROM  Table name ;

4. Four arithmetic queries

Query requirement

Check the file code

How to get there Navicat Import inside sql Data? ？

Start query operation

1. Query the specified field ename,job,sal The data of

SELECT ename,job,sal FROM emp;

2. Query all fields

SELECT empno,ename,job,mgr,hiredate,comm,deptno FROM emp;

Or use the following method to query all fields

① Not intuitive enough , You can't tell which fields there are at a glance

② Affect the efficiency of execution , First, go to the table to find which fields , And then check it out , There is one more step to find

--  Direct use is not recommended  * --
SELECT * FROM emp;

3. Query all positions of employees , And name it

SELECT ename,job FROM emp;

names

SELECT ename,job AS ' jobs ' FROM emp;

AS It can be omitted

SELECT ename,job ' jobs ' FROM emp;

4. Query the positions of employees ( Don't repeat )

SELECT job FROM emp;

Pay attention to the repetition , You need to display only one repetition

Use DISTINCT Keyword de duplication

SELECT DISTINCT job FROM emp;

If there are multiple fields , Can't get rid of the duplicate , This is the de duplication of a single field  

5. Check the annual salary of employees namely sal * 12

SELECT ename,sal AS ' a monthly salary ',sal * 12 AS ' Annual salary ' FROM emp;

SELECT sal / 10 FROM emp;
SELECT sal DIV 10 FROM emp;