Data query language (DQL)

Catalog

One 、 Basic query

1. Query single column data

2. Query multiple columns of data

3. Query all the data

4. De duplication query data

Two 、 Conditions of the query

1. Conditions of the query

2. Fuzzy query

3. Regular expressions

4. Sort and limit ：

3、 ... and 、 Multi-table query

1. The cartesian product

2. Equivalent connection and non equivalent connection

3. Internal and external connections

4. Left connection and right connection

5. Self join

6. Group query

7. Subquery

---

Inquire about Take the data out of the table , Put it in a temporary table .

There are three ways to query （ choice , Projection Select, Connect from union ）

The writing order of query statements is as follows

Select Clause from Clause

where Clause

order by Sort

having Clause

group by grouping

union Connect

The execution order of query statements is as follows

FROM WHERE GROUP BY HAVING SELECT ORDER BY

1． From First, determine the source of data retrieval --- surface

2． Where Determine the row corresponding to the qualification --- That's ok

3． Select Confirm that the data to be retrieved is projected to the corresponding column

It is known that student The database contains student,sc,course Three tables , The following example will query this table ：

One 、 Basic query

1. Query single column data

grammar ：

select  Name  from  Table name ;

Case study ：

Query student ID

select Sno from student;

2. Query multiple columns of data

grammar ：

select  Name 1, Name 2,..... Name n from  Table name ;

  Be careful ： Between column names, use ' , ' separate .

Case study ：

Check student number and student name

select Sno.Sname from student;

3. Query all the data

grammar ：

select * from  Table name ;

Case study ：

Query form student All data in

select * from student;

4. De duplication query data

grammar ：

select distinct  Name  from  Table name ;

  Be careful ： If the keyword is followed by more than one search column Then you need to retrieve all the data in the column to remove the duplicate .

Case study ：

Check the student's name

select distinct Sname from student;

Two 、 Conditions of the query

1. Conditions of the query

grammar ：

select  Name  from  Table name  where  Query criteria ;

Case study ：

see student The age in the table is 20 Student name of .

 

2. Fuzzy query

Fuzzy query keyword ：like;

% Indicates zero or more characters

_ Indicates that there is only one character

like Clause and where Equal sign of clause ‘=’ The effect is the same

grammar ：

select  Name  from  Table name  where  Name  like  Query criteria ;

Case study ：

Inquire about student The student surnamed Wang in the table .

 

3. Regular expressions

Mysql Regular expressions for use REGEXP Use this keyword to specify the matching pattern of the regular expression ; To add ' '.

Pattern	describe
^	Matches the start of the input string . If set RegExp Object's Multiline attribute ,^ Also match '\n' or '\r' The position after .
$	Matches the end of the input string . If set RegExp Object's Multiline attribute ,$ Also match '\n' or '\r' Previous position .
.	Matching elimination "\n" Any single character other than . To match includes '\n' Any character inside , Please use the image '[.\n]' The pattern of .
[...]	Character set . Matches any of the contained characters . for example , '[abc]' Can match "plain" Medium 'a'.
[^...]	Negative character set . Match any character not included . for example , '[^abc]' Can match "plain" Medium 'p'.
p1|p2|p3	matching p1 or p2 or p3. for example ,'z|food' Can match "z" or "food".'(z|f)ood' The match "zood" or "food".
*	Match previous subexpression zero or more times . for example ,zo* Can match "z" as well as "zoo".* Equivalent to {0,}.
+	Match previous subexpression one or more times . for example ,'zo+' Can match "zo" as well as "zoo", But can't match "z".+ Equivalent to {1,}.
{n}	n Is a non negative integer . Matched definite n Time . for example ,'o{2}' Can't match "Bob" Medium 'o', But it matches "food" Two of them o.
{n,m}	m and n All non negative integers , among n <= m. Least match n Times and at most m Time .

grammar ：
 

select  Name  from  Table name  where  Name  regexp ' Conditions ';

4. Sort and limit ：

Sort ： If we need to sort the read data , We can use MySQL Of ORDER BY Clause to set which fields and how you want to sort , Return to search results .

grammar ：

select  Query list  from  Table name  where  filter  order by  Sort list  asc / desc ;

asc: Ascending , If you don't write the default ascending order

desc: Descending

Case study ：

take sc The scores in the table are sorted in ascending and descending order

 

 

Limit ： You can limit the number of query results , stay MySQL Use in limit To limit the query .

grammar ：

select * from  Table name  limit  Number ;

Case study

Inquire about sc The top three scores in the table

 

3、 ... and 、 Multi-table query

1. The cartesian product

Each row of a table is connected with each row of the second table

grammar ：

select * from  surface 1, surface 2;

  Be careful ： When there are no restrictions on the connection of two meters , The connected table is N*M That's ok

Case study ：

take student and sc Two tables are connected by Cartesian product

 

2. Equivalent connection and non equivalent connection

The condition used to connect two tables in a join query is called a join condition , Its general format is ：

 Table name 1. Field name 1  Comparison operator   Table name 2. Field name 2;

Case study ：

Will student and sc Two tabular equivalent connections

Be careful ：“student s” Is for student The name of the table is , In order to make it more convenient to go to the equivalent condition later , In the equivalence condition , Because both tables contain sno, therefore , In order to distinguish the two, we should add the table name in front of it .

take student and sc Non equivalent connection in the table

 

 

3. Internal and external connections

Internal connection (inner join) Display the multi table connection results that meet the conditions , Empty lines that are all empty will not be displayed ;

By default, it is internal connection

External connection ( outer join) Display the multi table connection results that meet the conditions , All empty lines NULL It also shows .

Mysql External connection not supported OUTER JOIN

4. Left connection and right connection

LEFT JOIN（ Left connection ）： Get all the records in the left table , Even if the right table does not have a matching record .

Case study ：

 

RIGHT JOIN（ The right connection ）： And LEFT JOIN contrary , Used to get all records in the right table , Even if the left table does not have a matching record .

Case study ：

 

5. Self join

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

 

6. Group query

Grouping queries are done by adding aggregate functions to a with group by Grouping clause select Statement to achieve .

Use aggregate functions to query , Common aggregate functions are as follows ：

count([ DISTINCT | ALL] )： The number of records in the statistical table ;

count([ DISTINCT | ALL] < Name >)： The number of values in a column of the statistical data table ;

max([ DISTINCT | ALL] < Name >): Find the maximum value of a column of values in the data table ;

min([ DISTINCT | ALL] < Name >): Find the minimum value of a column of values in the data table ;

sum( DISTINCT | ALL] < Name >): Calculate the sum of the values in a column of the data table ;

avg( DISTINCT | ALL] < Name >): Calculate the average of the values in a column of the data table ;

group by The syntax format of the clause is ：

group by  Name  [having < Conditional expression >]

Case study ：

stay sc In the table grade grouping , Inquire about grade Greater than 90 The data of

 

7. Subquery

1. Subqueries are enclosed in parentheses

Case study ：

Query and “ Li Yong ” The name and Department of students in the same department

 

2.where Subquery in Clause , Subqueries can include grouping functions

        Subqueries can be found in where,having,from,update Of set Use in

        Grouping functions can be in select,having,group by Use in