SQL : Subquery without FROM clause - sql

Question

This is the table structure
CREATE TABLE Book_Tag (id INT, Book_Id INT, tag varchar(20))
CREATE TABLE Book_Master (Book_Id INT, Book_title VARCHAR(50), price INT)
And the data looks like this :
INSERT INTO Book_Master
SELECT 1, 'Good Profit', 28 UNION ALL
SELECT 2, 'The Secret', 20 UNION ALL
SELECT 3, 'The One Minute Manager', 9 UNION ALL
SELECT 4, 'The 7 Habits of Highly Effective People', 35 UNION ALL
SELECT null, 'Who Moved My Cheese?', 15 UNION ALL
SELECT null, 'Blink: The Power of Thinking Without Thinking', 40
INSERT INTO Book_Tag
SELECT 1, 1, 'Management' UNION ALL
SELECT 2, 1, 'Profit' UNION ALL
SELECT 3, 2, 'Mind' UNION ALL
SELECT 4, 3, 'Management' UNION ALL
SELECT 5, 3, 'Efficiency' UNION ALL
SELECT 6, 3, 'Success' UNION ALL
SELECT 7, 4, 'Success' UNION ALL
SELECT 8, null, 'Time' UNION ALL
SELECT 9, 6, 'SelfHelp' UNION ALL
SELECT 10, 6, 'Motivation' UNION ALL
SELECT 11, 8, 'Mind'
select * from Book_Master
Book_Id Book_title price
1 Good Profit 28
2 The Secret 20
3 The One Minute Manager 9
4 The 7 Habits of Highly Effective People 35
NULL Who Moved My Cheese? 15
NULL Blink: The Power of Thinking Without Thinking 40
select * from Book_Tag
id Book_Id tag
1 1 Management
2 1 Profit
3 2 Mind
4 3 Management
5 3 Efficiency
6 3 Success
7 4 Success
8 NULL Time
9 6 SelfHelp
10 6 Motivation
11 8 Mind
I dont know why the following is working and also why the result is that.
select BT.* from Book_Tag BT
where BT.Book_Id in (select id)
id Book_Id tag
1 1 Management
or this
select BT.* from Book_Tag BT
where BT.Book_Id not in (select id)
id Book_Id tag
2 1 Profit
3 2 Mind
4 3 Management
5 3 Efficiency
6 3 Success
7 4 Success
9 6 SelfHelp
10 6 Motivation
11 8 Mind

Answer 1

Your first query:
select BT.*
from Book_Tag BT
where BT.Book_Id in (select id);
It is the same as:
select BT.*
from Book_Tag BT
where BT.Book_Id = BT.id;
That is why you get
1 1 Management
Keep in mind that NULL is not equal NULL or anything else.
In second example you have:
select BT.*
from Book_Tag BT
where BT.Book_Id not in (select id);
Which is the same as:
select BT.*
from Book_Tag BT
where BT.Book_Id <> BT.id;
Note that there is no
8 NULL Time row.
EDIT:
But,In the Subquery shouldnt we be specifying the table from which the id is coming.
From MSDN:
The general rule is that column names in a statement are implicitly qualified by the table referenced in the FROM clause at the same level. If a column does not exist in the table referenced in the FROM clause of a subquery, it is implicitly qualified by the table referenced in the FROM clause of the outer query.
and
If a column is referenced in a subquery that does not exist in the table referenced by the subquery's FROM clause, but exists in a table referenced by the outer query's FROM clause, the query executes without error. SQL Server implicitly qualifies the column in the subquery with the table name in the outer query.

Answer 2

You're asking here "where BT.Book_Id in (select id)", is saying where Book_ID = ID. So in this case it prints that out. Book_ID and ID are both 1. Then not, would be when they aren't equal, which is everything else from that table.