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SQL OUTPUT PUZZLE | THE NULL and UNION – How well do you know them ? | SQL Interview Question

In this puzzle you have to go through all the SQL queries and provide us the output with really executing them at the SSMS.

Q1. What will be the OUTPUT of the following statement?

--

SELECT NULL
UNION ALL
SELECT NULL
UNION
SELECT NULL


--

SOLUTION

--

-----------
NULL

(1 row affected)

--

Reason – Here we will get a single NULL value because the operations will happen in top to bottom approach. So first UNION ALL will happen and after that UNION will happen and thats why we will get a single NULL value as the output.

Q2. What will be the OUTPUT of the following statement?

--

SELECT NULL
UNION 
SELECT NULL
UNION ALL
SELECT NULL

--

SOLUTION

--

-----------
NULL
NULL

(2 rows affected)

--

Reason – Here we will get two NULLs because the operations will happen in top to bottom approach. So first UNION will happen and after that UNION ALL will happen and thats why we will get two NULL values as the output.

Q3. What will be the OUTPUT of the following statement?

--

SELECT NULL
UNION 
SELECT NULL

--

SOLUTION

--

-----------
NULL

(1 row affected)

--

Reason – Here we a single NULL value. Although NULL is not equal to another NULL but in UNION all and UNION it behaves like they are equal. The same is the case with group by.

Enjoy 🙂

Please add comment(s) if you have one or multiple solutions in mind. Thank You.

Pawan Khowal

Pawan is a SQL Server Developer. If you need any help in writing code/puzzle or training please email at – pawankkmr”AT”gmail.com. Meanwhile please go throgh the top pages from his blog.

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