Advanced SQL statement 1 of Linux MySQL database

Experimental environment preparation

create table stu (id int,name varchar(10)primary key not null,score decimal(5,2),address varchar(20),hobbid int(5));

 insert into stu values(1,'zhao',100,'riben',2); insert into stu values(2,'qian',59.99,'taiguo',2); insert into stu values(3,'sun',75,'mianduan',3); insert into stu values(4,'li',99,'ouzhou',4); insert into stu values(5,'zhou',10,'huisuo',5); insert into stu values(6,'wu',11,'nanjin',3); insert into stu values(7,'zheng',80,'cv',3); insert into stu values(8,'wang',88,'cv',7);

One 、order by sentence

( increase 、 Delete 、 Change 、 check )
Yes MysQL Database query , In addition to basic queries , Sometimes you need to process the result set of the query . For example, take only 10 Data 、 Sort or group the query results, etc .
Sort by keyword
PS: Analogy to windows Task manager , Use SELECT Statement can transfer the required data from MySQL From the database , If you sort the results of the query , have access to ORDER BY Statement to sort statements , And finally return the sorted results to the user . The sorting of this statement can not only be for a certain field , You can also target multiple fields
grammar

SELECT column1,column2,... FROM table_name ORDER BY column1,column2,...

---- ORDER BY ---- Sort by keyword Syntax ：SELECT " Field " FROM " Table name " [WHERE " Conditions "] ORDER BY " Field " [ASC, DESC];#ASC  It's sorted in ascending order , Is the default sort method .#DESC  It's sorted in descending order .

1、 Single field sorting

order by It can also be combined with where Perform conditional filtration , The screening address is cv Of the students are ranked in descending order of scores

2、 Sorting multiple fields

Statements can also use multiple fields to sort , When there are multiple records with the same field in the first sorting , These multiple records are sorted according to the second field ,ORDER BY
When followed by multiple fields , The fields are separated by English commas , Priority is in order, but order by The next first parameter is only when the same value appears , The second field makes sense .
Query student information by interest first id Descending order , With the same score ,id Also in descending order

Query student information by interest first id Descending order , With the same score , id In ascending order

Two 、 Ascending and descending （ASC and DESC）

ASC：
It's sorted in ascending order , Is the default sort method , namely ASC It can be omitted .SELECT If no specific sorting method is specified in the statement , By default ASC Sort by .

DESC：
Is arranged in descending order . Of course ORDER BY You can also use WHERE Clause to further filter the query results .

3、 ... and 、select Inquire about

---- SELECT ---- Displays all data syntax for one or more fields in the table ：SELECT " Field " FROM " Table name ";

1、 View all

2、 View some fields

Four 、and/or- And / or

---- AND OR ---- And   Or grammar ：SELECT " Field " FROM " Table name " WHERE " Conditions 1" {[AND|OR] " Conditions 2"}+ ;

1、and

2、or

3、 nesting / Multiple conditions

5、 ... and 、distinct（ Query does not duplicate records ）

---- DISTINCT ---- Do not display duplicate data syntax ：SELECT DISTINCT " Field " FROM " Table name ";SELECT DISTINCT Store_Name FROM Store_Info;

6、 ... and 、where

--- WHERE ---- Conditional query syntax ：SELECT " Field " FROM " Table name " WHERE " Conditions ";

7、 ... and 、group by（ Group results ）

adopt SQL The result of the search , You can also group the total , Use GROUP BY Statement to implement ,GROUP BY Usually used in conjunction with aggregate functions , Common aggregation functions include : Count （ COUNT) 、 Sum up （SUM）、 Average （AVG）、 Maximum （MAX)、 minimum value （MIN),GROUP BY When grouping, the results can be grouped according to one or more fields .
grammar

SELECT column_name,aggregate_function (column_name)FROM table_name WHERE column_nameoperator valueGROUP BY column name;

---- GROUP BY ---- Yes GROUP BY Summarize and group the query results in the following fields , Usually used in combination with aggregate functions GROUP BY  There is a principle , Namely  SELECT  In all the following columns , Columns that do not use aggregate functions , Must appear in GROUP BY Back . grammar ：SELECT " Field 1", SUM(" Field 2") FROM " Table name " GROUP BY " Field 1";

1、 Press hobbid Same grouping , Calculate the number of students with the same score （ be based on name Count the number )

2、 combination where sentence , The filter score is greater than or equal to 80 The grouping , Count the number of students

3、 combination order by Arrange the number of students in ascending order
Transcript of the whole class count (name) :
Count score fraction :
score>=80 : good
score >=60 and score <80 : optimal -

select count(name),score,hobbid from stu where score >=80 group by  hobbid ;select count(name),score,hobbid from stu where score  >= 80 group by hobbid order by count(name) asc;

select count(name),score from stu where score > 80 group by score;

8、 ... and 、limit

Limit result entries ( limit )
limit Limit the result record of output
In the use of MySQL SELECT Statement to query , The result set returns all matching records （ That's ok ）. Sometimes you just need to return the first line or the first few lines , And that's where it comes in LIMIT Clause .
grammar

SELECT column1,column2,... FROM table_name LIMIT [offset, ] number

LIMIT The first parameter is the position offset （ Optional parameters ), It's settings MysQL Which line to start with . If the first parameter is not set , It will start from the first record in the table . It should be noted that , The position offset of the first record is 0, The second is 1, And so on . second : The first parameter is to set the maximum number of returned record rows .
Before all information is displayed 4 rows

combination order by sentence , Press id The first three lines are displayed in ascending order of size

Basics select How to output the last three lines for a small upgrade

Nine 、alias（ Set alias ）

Set alias ( alias ——》as )
stay MysQL When inquiring , When the name of the table is long or some fields in the table are long , For the convenience of writing or
Use the same table multiple times ," You can set aliases for fields, columns, or tables . Use aliases directly when using , Simple and clear , enhance readability
grammar

 For column aliases :SELECT column_name AS alias_name FROM table_name; For table aliases :SELECT column_name (s)FROM table_name As alias_name;

In the use of AS after , It can be used alias_name Instead of table_name, among AS The statement is optional .AS Later alias , It is mainly used to provide temporary names for columns or tables in the table , Use in the process of checking , The actual table name or segment name in the library will not be changed
Example of column alias setting :
select name as full name ,score as achievement from stu;
If the length of the table is long , have access to as Alias the table , In the process of query, the alias is directly used for temporary setting stu Its alias is i
select i.name as full name ,i.score as achievement from stu as i;
Inquire about stu Number of records in the table , With number Show

Use scenarios :
When querying complex tables , Aliases can shorten the length of query statements 2、 When querying multiple tables ( Easy to understand 、i Shorten sql sentence )
Besides ,as It can also be used as an operator of join statements .
establish t1 surface , take stu All query records of the table are inserted t1 surface

create table t1 as select * from stu;select * from t1

# here As What it does :
Created a new table t1 And define the table structure , Insert table data （ And info The table is the same )2、 however " constraint " Not completely " Copy “ To come over
# But if the primary key is set in the original table , Then the attached table : default The field will be set to one by default 0 be similar :
clone 、 Replicated table structure
create table t1 (select * from info);
# You can also join where Sentence judgment
create table test1 as select * from info where score >=60;
When setting an alias for a table , Make sure that the alias does not conflict with the names of other tables in the database .
The alias of the column is shown in the result , The alias of the table is not shown in the results , Use... Only when executing queries .
author ： Long feeling cl

Game programming , A game development favorite ~

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