In this post, I am sharing an advanced SQL Query to find a Permutations and Combinations of given string data of the table. The question is look like very complex query, but the solution is very simple.

You can apply self-joins and can prepare all available combinations. The T-SQL or Database Developer can practice this query before an interview.

Create a table with sample data:

CREATE TABLE tbl_StringAllCombinations (ID INT,TextData CHAR(1))
GO

INSERT INTO  tbl_StringAllCombinations VALUES 
(1,'A')
,(2,'B')
,(3,'C')
,(4,'D')
GO

The Solution: (This is my work around solution, Interviewer may ask for any other alternative solution so please also try to prepare it and share your comments…)

SELECT a.TextData, b.TextData, c.TextData, d.TextData
FROM tbl_StringAllCombinations a
join tbl_StringAllCombinations b on b.TextData not in (a.TextData)
join tbl_StringAllCombinations c on c.TextData not in (a.TextData, b.TextData)
join tbl_StringAllCombinations d on d.TextData not in (a.TextData, b.TextData, c.TextData)
order by 1, 2, 3, 4

The Result:

TextData TextData TextData TextData
-------- -------- -------- --------
A        B        C        D
A        B        D        C
A        C        B        D
A        C        D        B
A        D        B        C
A        D        C        B
B        A        C        D
B        A        D        C
B        C        A        D
B        C        D        A
B        D        A        C
B        D        C        A
C        A        B        D
C        A        D        B
C        B        A        D
C        B        D        A
C        D        A        B
C        D        B        A
D        A        B        C
D        A        C        B
D        B        A        C
D        B        C        A
D        C        A        B
D        C        B        A
(24 row(s) affected)

I have already shared hundreds of SQL Server Interview Questions.
Here, I would like to share one more common and important SQL Interview question for SQL Beginner.

Whenever I am taking the SQL Beginner’s Interview, I always ask one question “What is the logical order of SQL Query or SELECT Statement”.

It looks like a very basic question, but It is very important to know about the sequence of query execution because It plays main role in query optimization.

You should know that JOINS are executing the first or WHERE clause is executing the first. Both are filtering the data, but most of the time I have found a wrong answer.

Below is a correct logical order of SQL Query or SELECT Statement:

• FROM
• ON
• JOIN
• WHERE
• GROUP BY
• WITH CUBE or WITH ROLLUP
• HAVING
• SELECT
• DISTINCT
• ORDER BY
• TOP