Variables and SET statements in SQL - Step-by-Step Execution

Concept Flow - Variables and SET statements

Declare Variable

↓

Assign Value with SET

↓

Use Variable in Query or Logic

↓

Optionally Reassign with SET

↓

End or Use Variable Result

This flow shows how you declare a variable, assign it a value using SET, use it in queries or logic, and optionally change its value again.

Execution Sample

SQL

DECLARE @count INT;
SET @count = 5;
SET @count = @count + 3;
SELECT @count AS Result;

This code declares a variable, sets it to 5, adds 3, then selects the final value.

Execution Table

Step	Action	Variable	Value	Output/Result
1	Declare variable @count	@count	NULL	No output
2	SET @count = 5	@count	5	No output
3	SET @count = @count + 3	@count	8	No output
4	SELECT @count AS Result	@count	8	Result = 8
5	End of script	-	-	Execution stops

💡 Script ends after SELECT statement returns the variable value.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
@count	NULL	5	8	8

Key Moments - 3 Insights

Why is the variable @count NULL right after declaration?

When declared, variables have no value until SET assigns one, as shown in step 1 and 2 of the execution_table.

What happens if you try to use @count before SET assigns a value?

It will be NULL or cause an error depending on context, because variables start without a value until SET assigns one (see step 1).

Can you change the value of a variable after it is set?

Yes, you can reassign it with SET as shown in step 3 where @count changes from 5 to 8.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of @count after step 2?

A NULL

B 8

C 5

D 0

💡 Hint

Check the 'Value' column in row for step 2 in execution_table.

At which step does @count get its first assigned value?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for the first SET statement assigning a value in execution_table.

If you remove step 3, what would be the output at step 4?

A 5

B NULL

C 8

D Error

💡 Hint

Without step 3, @count remains as assigned in step 2; check variable_tracker for values.

Concept Snapshot

DECLARE variable_name datatype;  -- creates a variable
SET variable_name = value;        -- assigns or changes value
Variables start NULL until SET assigns a value
Use variables in queries or logic by referencing their name
You can reassign variables multiple times with SET

Full Transcript

This lesson shows how to use variables in SQL. First, you declare a variable with DECLARE, which creates it but does not give it a value yet. Then you assign a value using SET. You can change the value later by using SET again. Finally, you can use the variable in queries or logic. The example code declares @count, sets it to 5, adds 3 to make 8, and then selects the result. The execution table shows each step and how the variable changes. Beginners often wonder why the variable is NULL at first or if they can change it later. The answer is yes, you must assign it before use, and you can reassign it anytime. The quiz questions help check understanding of when and how the variable changes.