Mysql database easy learning 06 - commonly used by data analysts: single table query of data query language DQL

Data query language DQL

Used to query records in database tables
The basic structure ：

SELECT  Field name  FROM  Table or view name  WHERE  Query criteria 

Single table query

1、 The result is a virtual result set

select After the statement is executed, the server will retrieve the data in the table as required , And send the search results to the client , This temporary result set in the form of a table , It is Store in memory , Not on disk , After performing other operations, the result set is gone , So it is A temporary virtual result set , Not a real table .

2、 A full table query

select * from  Table name ; 

If a table has a large volume , Generally, the following two methods are used to view the data in the table

1、 Choose the former 10 Row to view top 10

select top 10 *  from  Table name 

2、 Limit the number of query results limit

select  Field 1[, Field 2,…] from  Table name  limit [ Offset ,]  Row number ; 

limit Accept One or two numbers Parameters , Argument must be an integer constant
The first parameter specifies the offset of the first return record line , The second parameter specifies the maximum number of rows to return records
If only one parameter is given , Indicates the maximum number of record lines returned
The offset of the initial record line is 0( instead of 1)
Example ：
dept There is a total of 4 Row data

3、 Query specified column

select  Field 1[, Field 2,…] from  Table name ; 

4、 Query non duplicate data distinct

select distinct  Field name  from  Table name ; 

5、 Set alias as

select  Field name  as  Column alias  from  Original table name  [as ] Table alias ;

6、 Conditions of the query where

select  Field 1[, Field 2,…] from  Table name  where  filter ; 

You can add... To the filter criteria Operator ：
Arithmetic operator :
+ Add ; - reduce ;* ride ; / except

Comparison operator :
= be equal to ;>/>= Greater than / Greater than or equal to ;</<= Less than / Less than or equal to ;
!=/<> It's not equal to ;
between…and… Between what

Logical operators
and And ;or or ;not Not

7、 Null value query is null

select  Field 1[, Field 2,…] from  Table name  where  Null value field  is[not] null; 

Be careful ： Null values cannot be compared , If you want to filter out null values, you should use is null

8、 Fuzzy query like wildcard

select  Field 1[, Field 2,…] from  Table name  where  String field [ not] like  wildcard ; 

wildcard ：
Percent sign （%） wildcard ： matching 0 Characters or more
Underline （_） wildcard ： Match a character

9、 Aggregate operations

Aggregate multiple rows of data into one row
The aggregate function will do null Other data Aggregate operations

10、 Grouping query and grouping filter

Group query ：

select  Field 1[, Field 2,…] from  Table name [ where  Query criteria ] group by  Grouping field 1[, Grouping field 2,…];

Group query results by one or more fields , The fields with the same value are a set of , Aggregate each group
When grouping multiple fields , Multiple fields with exactly the same value will be divided into a group
Filter after grouping :

select  Field 1[, Field 2,…] from  Table name [ where  Query criteria ][ group by  Grouping field 1[, Grouping field 2,…]] having  filter ; 

where And having The difference between ：
where In grouping and aggregation Computing Before Filter the data in the table , and having In grouping and aggregation after Filter the divided groups , therefore where Clause cannot contain aggregate function .

11、 Sorting of query results order by

select  Field 1[, Field 2,…] from  Table name  order by  Field 1[  Sort direction , Field 2  Sort direction ,…]; 

Sort query results by one or more fields , When sorting, you can specify the sorting direction for each field . When sorting multiple fields , First, sort by the first field , If the first field has the same value, sort by the second field
Specify sorting direction ：
asc（ ascendent） Ascending , for example [ 0 1 2 3 4 5 ];
desc（descendent） Descending , for example [ 5 4 3 2 1 0 ]