How to use the select statement of MySQL

MySQL Of select How to use statements

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select Statement can To separate... With carriage return

$sql="select * from article where id=1" and   $sql="select * from article where id=1" Can get the right results , But sometimes writing separately may   More clearly , Especially when sql When the statement is long .

Bulk query data

 It can be used in  To achieve $sql="select * from article where id  ;in(1,3,5)"

Use concat Connect the results of the query

$sql="select concat(id,"-",con)  as res from article where id=1" return  "1-article content"

Use locate

 usage ： select locate("hello","hello baby"); return 1
 There is no return  0

Use group by

 I haven't figured it out before group by   and  order by, It's actually quite simple ,group by  Is to tie the same knot   Make the fruit into a group 
exam：$sql="select city ,count(*)  from customer group by city";
 The meaning of this sentence is   Thinking is from customer The table lists all cities that do not repeat , And quantity （ It's kind of similar distinct）
group by  Often with AVG(),MIN(),MAX(),SUM(),COUNT() Use it together 

Use having

having  Allow conditional aggregation of data into groups $sql="select city,count(*),min(birth_day)  from customer
group by city having c ount(*)>10";
 This sentence is to press city Grouping , Then find out city The quantity of land is greater than 10 The city of   City 
btw： Use group by + having  It's a little slow 
 Same as   when having Clause must contain an expression that has occurred before 

Combined clause

where、 group by、having、order by（ If you want to use all four , Usually press this   In one order ）

Use distinct

distinct It is used to remove duplicate values $sql="select distinct city from customer  order by id desc";
 This sentence means from customer Look up in the table   Ask for all non repetitive city

Use limit

 If you want to display the information after a record   Records $sql="select * from article limit 100,- 1";

Multi-table query

$sql="select user_name from user  u,membe r  m
where u.id=m.id and
m.reg_date> ;=2006-12-28
order by u.id desc"

Be careful ： If user and member Both labels have user_name Field , There will be mysql error （ because mysql I didn't know you were there Which table do you want to query user_name）, You must indicate which table it belongs to .

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