Simplify Self Join SQL

Question

I am currently doing the following join and it takes a very long time to execute the query, is there a way to make it run quicker? I have 23 fields in the table. I have a primary key on the field 'id' (int) 5 reg (varchar) 9

SELECT a1.*, ( SELECT COUNT(reg) FROM auction a2 WHERE reg = a1.reg) AS c1 FROM auction a1


CREATE TABLE IF NOT EXISTS `db1` (
  `id` int(5) unsigned NOT NULL AUTO_INCREMENT,
  `adate` varchar(10) NOT NULL,
  `lnu` int(4) NOT NULL,
  `reg` varchar(9) NOT NULL,
  `mk` varchar(50) NOT NULL,
  `mod` varchar(50) NOT NULL,
  `type` varchar(25) NOT NULL,
  `ree` varchar(50) NOT NULL,
  `co` varchar(50) NOT NULL,
  `fu` varchar(20) NOT NULL,
  `tran` varchar(20) NOT NULL,
  `mt` varchar(9) NOT NULL,
  `mile` int(6) NOT NULL,
  `ree` int(6) NOT NULL,
  `apv` int(6) NOT NULL,
  `ds` int(2) NOT NULL,
  `sr` varchar(50) NOT NULL,
  `sen` int(10) NOT NULL,
  `keep` int(2) unsigned NOT NULL,
  `tmp` char(1) DEFAULT NULL,
  `rk` varchar(100) DEFAULT NULL,
  `st` varchar(3) DEFAULT NULL,
  `dd` varchar(10) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `date` (`adate`)

Answer 1

如前所述，如果您唯一的索引是（id，reg）上的主键，那么通过单独添加（reg）上的索引将大大加快此查询的速度。

Answer 2

Seeing fromyour current schema, you should have provided index on reg column.

SELECT a1.*, a2.totalCount
FROM    auction a1
        INNER JOIN 
        (
            SELECT reg, COUNT(*)  totalCount
            FROM auction a2
            GROUP BY reg
        ) a2 
            ON a2.reg = a1.reg

Answer 3

Have you tried using a sub-query:

SELECT a1.*, a2.cnt
FROM auction a1
INNER JOIN
(
    SELECT COUNT(reg) cnt, reg
    FROM auction
    GROUP BY reg
) a2
    on a1.reg = a2.reg

Answer 4

You have 2 options:

1. Have an index on the join column ie reg . This may help fasten the query.

2. Use CTE.

With CTE001 AS ( Select count( ) as GroupCount, reg from auction group by reg ) Select a. , c.GroupCount from CTE001 c inner join auction a on c.reg = a.reg;