The role of the symbol @ in MySQL

Catalog

mysql Of @ usage

actual combat

select @a;

Variable name , If you don't , You will think that this is a column name , But this column doesn't exist , It's a mistake ;

@ Variable name : Define a user variable .

= Assign a value to the user variable .

There are two ways to assign user variables : One is direct use "=“ Number , The other is to use ”:=“ Number .

The difference is :

Use set When the command assigns a value to a user variable , Both methods can be used ;

use select When the sentence is , Only use ”:=“ The way , because select In the sentence ,”=" The sign is considered a comparison operator .

(@i:[email protected]+1)

You can generate a set of incremental serial numbers when generating the query result table

select (@i:[email protected]+5) as rownum, surname, personal_name from student, (select @i:=100) as init;

select @ids := 101,@l := 0

GROUP_CONCAT + group by

Group by , Connect the data on the field , Default to ,, You can also specify a delimiter

mysql Of @ usage

1, Add temporary table , Self increment of variables

SELECT (@i:[email protected]+1),t.* FROM table_name t,(SELECT @i:=0) AS j

(@i:[email protected]+1) Represents the definition of a variable , Every time you stack 1;
(SELECT @i:=0) AS j For creating a temporary table ,j Is a random table name , But aliases must be .

2, Implement sort increment

SELECT( @i := @i + 1 ),p.* FROM( SELECT * FROM sys_region ORDER BY create_time DESC ) p, ( SELECT @i := 0 ) k

3, Realize group increment

[email protected] := CASE WHEN @type = a.LEVEL THEN @r + 1 ELSE 1 END AS rowNum,@type := a.`level` AS type,a.id FROMsys_region a,( SELECT @r := 0, @type := '' ) b;

Temporary variables are used here @type, Because when operating on temporary variables , The variables used are all temporary variables .

actual combat INSERT INTO t_top ( col_account, col_date, p_account, p_name )SELECT b.col_account, b.col_date, b.p_account, b.p_name FROM([email protected] := CASE WHEN @account = a.col_account THEN @num + 1 ELSE 1 END AS rownum,@account := a.col_account AS account,a.* FROM( SELECT * FROM zb_top ORDER BY col_account, p_avg DESC ) a,( SELECT @num := 0, @account := '' ) j) b where 6 > b.rownum

Calculate the number of days since the user last visited , according to imei Number to distinguish different users , If there is only one visit in the period, it is 0.

Initialization data ：
Code example :

CREATE TABLE `pd` (`imei` varchar(32) NOT NULL DEFAULT '',`date` datetime DEFAULT NULL) ENGINE=InnoDB DEFAULT CHARSET=utf8;INSERT INTO `pd` VALUES ('1', '2013-07-25 00:00:01');INSERT INTO `pd` VALUES ('1', '2013-07-26 00:00:02');INSERT INTO `pd` VALUES ('2', '2013-07-23 00:00:04');INSERT INTO `pd` VALUES ('2', '2013-07-26 00:00:03');INSERT INTO `pd` VALUES ('3', '2013-07-26 00:00:01');

Script , Use @ Special variables ：
Code example :

SELECT * FROM(SELECTimei user_id,max(max_dd),max(max_dd_2),to_days(max(max_dd)) - to_days(max(max_dd_2)) days FROM(SELECTimei,max_dd,max_dd_2 FROM(SELECTtmp.imei,tmp.date,IF(@imei = tmp.imei, @rank := @rank + 1, @rank := 1) AS ranks,IF(@rank = 1, @max_d := tmp.date, @max_d := NULL) AS max_dd,IF(@rank = 2, @max_d_2 := tmp.date, @max_d_2 := NULL) AS max_dd_2,@imei := tmp.imei FROM(SELECT imei, date FROM pd ORDER BY imei ASC, date DESC) tmp,(SELECT @rownum := 0, @imei := NULL, @rank := 0, @max_d := NULL, @max_d_2 := NULL) a ) result ) t GROUP BYimei HAVINGcount(*) > 1 ) x WHERE x.days >= 1 AND EXISTS (SELECT 'x' FROM pd WHERE date > '2013-07-26 00:00:00')

Be careful ：
When there is a large amount of data in the table , Use union all Such operations will have tragic results .

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