condition record exists but has not set

1. Exists's implementation process SELECT * from t1 where exists (select null from t2 where y = x) Can be understood as: Copy Code code as follows: For x in (SELECT * from T1) Loop if (exists (select null from t2 where y = x.x)

SQL query in the in, exists, not in, not exists usage and difference, sqlexists 1. in and exists In connects the External table to the internal table as a hash (Dictionary set), while exists loops the External table and queries the internal table

Usage and difference of in, exists, not in, and not exists in SQL queries 1. in and exists In connects the External table to the internal table as a hash (Dictionary set), while exists loops the External table and queries the internal table after

from the point of view of efficiency: 1 SELECT * from T1 where exists (select 1 from T2 where t1.a=t2.a); When the amount of T1 data is small and the T2 data is very large, the query efficiency of the T1 2) SELECT * from T1 where t1.a in (select t2

1. Execution Process of EXISTS Select * from t1 where exists (select null from t2 where y = x)It can be understood:Copy codeThe Code is as follows:For x in (select * from t1)LoopIf (exists (select null from t2 where y = x. x)ThenOUTPUT THE RECORDEnd

In is the appearance and the inner table as a hash connection, and exists is the external loop loop, each loop loop and then query the internal table.NOT exists: Do NL, subquery first, there is a virtual table, there is a definite value, so even if

Original: MySQL not in, left join, is NULL, not EXISTS efficiency problem recordMySQL not in, left joins, is NULL, not EXISTS efficiency problem records, the need for friends can refer to.No in , JOIN, is NULL, not exists efficiency

First of all declare: Database master Please go away, you saw an eyesore ah. Sometimes when the query, Ah, a field in, and not in simply can not meet the requirements, very much like more than one field in the, not in, but many databases do not

Content from online:53090467The data of the main query is placed in the subquery for conditional validation, depending on the validation result (TRUE or FALSE) to determine whether the data results of the main query are retained.Here's an example of

    In MSSQL, inserting a record is simple, but in some special applications, before inserting a record, you need to check whether the record exists. The insert operation is performed only when the record does not exist, this article describes the