MySQL - Basic select statement

1、DQL（Data Query Language： Data query language ）

• All query operations use it SELECT
• The core language in database , The most important statement
• The most frequently used statement

2、 Specify query fields

SELECT * FROM  Table name ;	--  Query all field information in the table 

SELECT  Field 1, Field 2 FROM  Table name ;	--  Query all the information of the specified field 

SELECT  Field 1 AS  Alias 1, Field 2 AS  Alias 2 FROM  Table name ;		--  Alias fields （ You can also alias a table ）

SELECT CONCAT(' character string 1',' character string 2',···) ;		--  String concatenation 

SELECT DISTINCT  Field name 1 FROM  Table name ;		--  Duplicate data found , duplicate removal 

3、where Conditional clause

• effect ： Retrieve data eligible Value
• Logical operators ：

  Operator	grammar	describe
  and / &&	a and b / a&&b	Logic and , All true , The result is true
  or / ||	a or b / a||b	Logic or , One of them is true , The result is true
  not / !	not a / !a	Logic is not , True or false , False is true

• Fuzzy query ：

  Operator	grammar	describe
  IS NULL	a is null	If the operator is NULL, The result is true
  IS NOT NULL	a is not null	If the operator is not NULL, The result is true
  BETWEEN	a between b and c	if a stay b and c Between , The result is true
  LIKE	a like b	SQL matching , If ah matches b, The result is true
  IN	a in (a1,a2,a3···)	hypothesis a stay a1 perhaps a2··· One of the values , The result is true

4、 League table query

operation	describe
inner join	If there is at least one match in the table , Just go back
left join	All values are returned from the left table , Even if there is no match in the right table
right join	All values are returned from the right table , Even if there is no match in the left table

• Connect your own table to your own table , The core ： One table can be split into two identical tables

  SELECT a. Field name  AS ' Parent column ' b. Field name  AS ' Sub column '
  FROM  Table name 1 AS a,  Table name 1 AS b
  WHERE a. Field name 1 = b. Field name 2;
  

5、 Paging and sorting

• Sort ：ORDER BY

  SELECT  Field name 1, Field name 2,··· FROM  Table name 
  ORDER BY  Field name 1 ASC/DESC[, Field name 2 ASC/DESC,···];
  

• Pagination ：LIMIT

  SELECT * FROM  Table name  LIMIT  Starting value , Page size ;
  

6、 Subquery

• where The latter condition is select A result set ：

  SELECT  Field name  FROM  Table name  WHERE  Field name 1 = 
  (SELECT  Field name 1 FROM  Table name  WHERE  Field name 1=  Conditional expression );
  

7、 Aggregate functions

SELECT  Field name 1,AVG( Field name 1) FROM  Table name  GROUP BY  Field name 1 [HAVING  Conditional expression ];