1.有2張相同的表,字段相同,數據大概相同,有20w條數據,當有一張表數據缺失時,怎麼查找出缺失的數據?(not exists)
至於20w條數據,可以給name加個索引提升速度
SELECT * FROM `a` WHERE not EXISTS (SELECT * FROM `b` WHERE a.name=b.name)
2.查詢一張表中重複的數據(group by,count)
SELECT name,count(*) as count FROM `a` GROUP by name HAVING count>1