SQL Server Management Studio中的查询优化

Question

In the query given below, in the where condition the table 'a' is not accessible. How can I make it accessible without affecting the logic of the code?

Thanks in advance.

select * 
from Training_Data.dbo.test a
cross apply (select * from Training_data.dbo.test_table) b
where exists (select 1 from a)

Answer 1

To make the table accessible use real table name instead of alias.

select * 
from Training_Data.dbo.test a
 cross apply (select * from Training_data.dbo.test_table) b
where exists (select 1 from Training_Data.dbo.test)

By the way I suppose that it would be more appropriate to check existence of the data in the table before the select in IF clause and to use cross join instead of cross apply

if exists (select 1 from #T1)
 select *
 from #T1 a
  cross join #T1 b

Answer 2

You can access your referenced tables columns in any EXIST inside your WHERE , you just need a correct syntax.

Example: SELECT if there's a record with same value in another table (reference to a column from table test :

select 
    * 
from 
    Training_Data.dbo.test a
    cross apply (select * from Training_data.dbo.test_table) b
where 
    exists (
        select 
            1 
        from 
            Training_data.dbo.another_test_table c 
        where 
            a.someColumn = c.anotherColumn)

Example: SELECT if there are records on a table (without referencing any column):

select 
    * 
from 
    Training_Data.dbo.test a
    cross apply (select * from Training_data.dbo.test_table) b
where 
    exists (
        select 
            1 
        from 
            Training_Data.dbo.test c)

For this last example, if you want to select rows from a table only if they exist in the same table, then your SELECT will already handle that, since it will fetch the rows that are in that table.

Answer 3

A programmer can use an alias to temporarily assign another name to a table or column for the duration of a SELECT query. Assigning an alias does not actually rename the column or table. This is often useful when either tables or their columns have very long or complex names. Hope you got your answer. You can put condition using column name if you don't want to change anything, but you can't use alias as table.