MySQL Query Statement Execution order

MySQL query statements are divided into 11 steps, each of which will produce a virtual table, the virtual table as a processing input, but these virtual tables are transparent to the user, but only the last virtual table will be returned as the result.

Statement execution Order:

Order	Name	Content
1	SELECT	Command initiation
2	* or AVG (field), etc.	Query content
3	From	Query location
4	WHERE	Conditional query
5	GROUP by	Group
6	Having	Screening
7	ORDER by (ASC/DESC)	Sort
8	LIMIT	Limit the number of results

1. Single-table query

• Querying the specified column

-- Check the student's form for the number and name; Select  from student;

• Query all Columns

-- Check all information in student tables   from  student;

• To alias a column after a query

-- Check the "name" and "Year of birth" columns for all students Select  as name, as from student;

• Single-Condition Query

-- query position is a  record of salesman;Select*fromwhere job='  Salesman'

• Multi-Criteria Query

--Query department records in 10 or 20Select *  fromEmpwhereDeptno=Ten orDeptno= -;--Query Department in 10 or department at 20 and salary less than 2000 recordSelect *  fromEmpwhereDeptno=Ten or(Deptno= -  andSal<= -);

Go to re-query, use <DISTINCT column name > statement

-- Check the student number of the course Select distinct  as  from score;

• Query null value (NULL)

 --  query No course number and course name for the first course.  select  Cno as  course number, Cname as  course name, cpno from  course  where  cpno is  null  ;  --  Check all students with grades, course numbers and grades  select  Sno as , Cno as  course number, Grade as  score from  SC where  Grade is  not  null ; 

• Convert NULL to actual value, using <COALESCE (column name, replace value) > statement

-- query allowance is null, replaced by 0 Select COALESCE (Comm,0 from EMP;

• The fuzzy query (like% _ statement)% denotes any character; _ Represents a single character

--query employee names starting with a recordSelect *  fromEmpwhereEname like 'a%';--Query the employee name for a record that contains aSelect *  fromEmpwhereEname like '%a%';--Query Employee Name The second letter begins with a recordSelect *  fromEmpwhereEname like '_a%';

MySQL Query Statement Execution order