WITH clause syntax in SQL - Step-by-Step Execution

Concept Flow - WITH clause syntax

Start Query

↓

Define CTE with WITH

↓

Use CTE in main query

↓

Execute main query

↓

Return results

↓

End

The WITH clause defines a temporary named result (CTE) used by the main query that follows.

Execution Sample

SQL

WITH SalesCTE AS (
  SELECT ProductID, SUM(Quantity) AS TotalSold
  FROM Sales
  GROUP BY ProductID
)
SELECT * FROM SalesCTE WHERE TotalSold > 100;

This query defines a CTE named SalesCTE that sums quantities sold per product, then selects products with sales over 100.

Execution Table

Step	Action	Evaluation	Result
1	Parse WITH clause	Define SalesCTE as grouped sum of Quantity by ProductID	SalesCTE ready with aggregated data
2	Execute CTE query	Calculate SUM(Quantity) per ProductID	Temporary table SalesCTE created
3	Execute main query	Select rows from SalesCTE where TotalSold > 100	Filtered rows with TotalSold > 100
4	Return results	Output filtered rows	Final result set returned
5	End	No more steps	Query execution complete

💡 Query ends after returning filtered results from the CTE

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
SalesCTE	undefined	Aggregated data by ProductID	Filtered rows with TotalSold > 100	Filtered result set

Key Moments - 3 Insights

Why do we define the CTE before the main query?

The CTE must be defined first (see execution_table step 1) so the main query can use its results.

Is the CTE data stored permanently?

No, the CTE is temporary and only exists during query execution (execution_table step 2).

Can the main query use the CTE multiple times?

Yes, the CTE acts like a temporary table accessible throughout the main query (execution_table step 3).

Visual Quiz - 3 Questions

Look at the execution table, what happens at step 2?

A The CTE query calculates aggregated sums

B The main query filters rows from SalesCTE

C The query ends and returns results

D The WITH clause is parsed but not executed

💡 Hint

Check the 'Action' and 'Evaluation' columns at step 2 in the execution_table

At which step does the query return the final results?

A Step 1

B Step 3

C Step 4

D Step 5

💡 Hint

Look for 'Return results' in the 'Action' column of execution_table

If the WHERE clause in the main query was removed, how would the variable SalesCTE change after step 3?

A It would be empty

B It would contain all aggregated rows without filtering

C It would contain only rows with TotalSold <= 100

D It would cause an error

💡 Hint

Refer to variable_tracker and execution_table step 3 about filtering

Concept Snapshot

WITH clause syntax:
WITH cte_name AS (
  subquery
)
SELECT * FROM cte_name;
- Defines a temporary named result (CTE)
- CTE is used by the main query
- CTE exists only during query execution

Full Transcript

The WITH clause in SQL lets you define a temporary named result called a Common Table Expression (CTE). First, the CTE is defined with a subquery that runs and creates a temporary table. Then, the main query uses this CTE as if it were a regular table. The CTE only exists during the query execution and helps organize complex queries by breaking them into parts. In the example, SalesCTE sums quantities sold per product. The main query then selects products with total sales over 100. The execution steps show parsing the WITH clause, running the CTE subquery, filtering results in the main query, and returning the final output.

Practice

1. What is the main purpose of the WITH clause in SQL?

easy

A. To create temporary named result sets called Common Table Expressions (CTEs)

B. To permanently store data in a new table

C. To delete rows from a table

D. To update values in a table

Solution

1. Step 1: Understand the role of WITH clause

   The WITH clause defines temporary named result sets called CTEs that exist only during query execution.

2. Step 2: Compare options with WITH clause purpose

   Options B, C, and D describe permanent or data modification actions, which are not the purpose of WITH.

3. Final Answer:

   To create temporary named result sets called Common Table Expressions (CTEs) -> Option A

4. Quick Check:

   WITH clause = Temporary named results [OK]

Hint: WITH clause defines temporary named queries [OK]

Common Mistakes:

• Thinking WITH creates permanent tables
• Confusing WITH with data modification commands
• Assuming WITH is used for deleting or updating data

2. Which of the following is the correct syntax to start a query using a single CTE named cte_example?

easy

A. CREATE cte_example AS (SELECT * FROM table_name); SELECT * FROM cte_example;

B. WITH (SELECT * FROM table_name) AS cte_example SELECT * FROM cte_example;

C. WITH cte_example AS (SELECT * FROM table_name) SELECT * FROM cte_example;

D. SELECT * FROM cte_example WITH (SELECT * FROM table_name);

Solution

1. Step 1: Recall correct WITH clause syntax

   The correct syntax is: WITH cte_name AS (subquery) followed by the main SELECT using cte_name.

2. Step 2: Check each option's syntax

   WITH cte_example AS (SELECT * FROM table_name) SELECT * FROM cte_example; matches the correct syntax. The other options misuse keywords or order.

3. Final Answer:

   WITH cte_example AS (SELECT * FROM table_name) SELECT * FROM cte_example; -> Option C

4. Quick Check:

   WITH cte_name AS (query) SELECT ... [OK]

Hint: WITH cte_name AS (query) SELECT ... is correct syntax [OK]

Common Mistakes:

• Using CREATE instead of WITH for CTE
• Placing AS after the subquery instead of after CTE name
• Misordering clauses in the query

3. Given the following SQL query, what will be the output?

WITH numbers AS (SELECT 1 AS num UNION ALL SELECT 2 UNION ALL SELECT 3) SELECT SUM(num) FROM numbers;

medium

A. 1

B. 3

C. Syntax error

D. 6

Solution

1. Step 1: Understand the CTE content

   The CTE named 'numbers' contains rows with values 1, 2, and 3.

2. Step 2: Calculate the SUM of num column

   Sum of 1 + 2 + 3 equals 6.

3. Final Answer:

   6 -> Option D

4. Quick Check:

   1 + 2 + 3 = 6 [OK]

Hint: Sum values inside CTE to get result [OK]

Common Mistakes:

• Assuming UNION ALL removes duplicates
• Confusing SUM with COUNT
• Expecting syntax error due to CTE

4. Identify the error in this SQL query using a CTE:

WITH cte1 AS SELECT * FROM employees SELECT * FROM cte1;

medium

A. CTE name cannot be cte1

B. Missing parentheses around the CTE's SELECT statement

C. SELECT * FROM cte1 should come before WITH clause

D. WITH clause cannot be used with SELECT

Solution

1. Step 1: Check CTE syntax

   The CTE definition must have parentheses around the SELECT query: AS (SELECT ...)

2. Step 2: Identify missing parentheses

   The query misses parentheses after AS, causing syntax error.

3. Final Answer:

   Missing parentheses around the CTE's SELECT statement -> Option B

4. Quick Check:

   CTE query must be in parentheses [OK]

Hint: Always put CTE query inside parentheses after AS [OK]

Common Mistakes:

• Omitting parentheses around CTE query
• Placing main SELECT before WITH clause
• Thinking CTE names have restrictions beyond naming rules

5. Consider this SQL query using multiple CTEs:

WITH cte1 AS (SELECT id, salary FROM employees WHERE department = 'Sales'), cte2 AS (SELECT id, bonus FROM bonuses) SELECT cte1.id, cte1.salary, cte2.bonus FROM cte1 LEFT JOIN cte2 ON cte1.id = cte2.id;

What is the purpose of using multiple CTEs here?

hard

A. To organize complex queries by separating filtering and joining steps

B. To permanently store intermediate results in the database

C. To speed up query execution by caching results automatically

D. To avoid using JOINs by combining data in CTEs

Solution

1. Step 1: Analyze the role of each CTE

   cte1 filters employees in Sales with id and salary; cte2 selects bonuses by id.

2. Step 2: Understand the main query usage

   The main query joins these CTEs to combine salary and bonus data for Sales employees.

3. Step 3: Identify why multiple CTEs are used

   Using multiple CTEs organizes the query into clear steps: filtering and joining, improving readability.

4. Final Answer:

   To organize complex queries by separating filtering and joining steps -> Option A

5. Quick Check:

   Multiple CTEs = Organize complex queries [OK]

Hint: Use multiple CTEs to break complex queries into parts [OK]

Common Mistakes:

• Thinking CTEs store data permanently
• Assuming CTEs automatically speed up queries
• Believing CTEs replace JOINs entirely