php - 如何加速笛卡尔的 MySQL 查询

Question

假设有一个名为 contact 的表，其结构如下:

id INT -- primary key,autoincrement,index 
firstname VARCHAR (255),
lastname VARCHAR(255),
type ENUM

执行这样的查询:

SELECT c1.id AS c1_id, c2.id AS c2_id
FROM contact c1
INNER JOIN contact c2 ON c1.firstname = c2.firstname AND c1.lastname = c2.lastname
WHERE c1.id <> c2.id AND c1.type=c2.type

在少量记录上没问题......但是当记录数从 30-40 增长到 1000 时，这个查询非常慢。需要从记录数中抽象出来并尽可能加快这个查询。任何建议？

Answer 1

你可以试试这样的:

SELECT GROUP_CONCAT(DISTINCT id), firstname, lastname
FROM contact
GROUP BY firstname, lastname
HAVING COUNT(DISTINCT id)>1

这将返回所有重复的名称。如果您想要 ID，则可以使用 JOIN:

SELECT
  contact.id
FROM
  contact INNER JOIN (SELECT firstname, lastname
                      FROM contact
                      GROUP BY firstname, lastname
                      HAVING COUNT(DISTINCT id)>1) dup
  ON contact.firstname=dup.firstname AND contact.lastname=dup.lastname