For loops for dynamic SQL in dbt

Introduction

For loops help you create SQL code that changes based on your data. This saves time and avoids mistakes when writing similar SQL many times.

You want to create multiple similar SQL queries for different tables or columns.

You need to build a report that shows data for many categories without writing each query by hand.

You want to automate SQL code generation to handle changing data structures.

You need to repeat a SQL pattern for a list of values or dates.

You want to keep your SQL code clean and easy to update.

Syntax

dbt

{% for item in list %}
  -- your SQL code using {{ item }}
{% endfor %}

Use {% for %} and {% endfor %} to start and end the loop.

Inside the loop, use {{ item }} to insert the current value.

Examples

This creates three SELECT statements, one for each column name.

dbt

{% for col in ['sales', 'profit', 'cost'] %}
  SELECT {{ col }} FROM my_table
{% endfor %}

This loops over days 1 to 3 and creates WHERE clauses for each date.

dbt

{% for day in range(1, 4) %}
  WHERE date = '2024-06-0{{ day }}'
{% endfor %}

Sample Program

This code sums up sales, profit, and cost columns in one query. The loop creates each SUM part and adds commas except after the last one.

dbt

{% set columns = ['sales', 'profit', 'cost'] %}

SELECT
{% for col in columns %}
  SUM({{ col }}) AS total_{{ col }}{% if not loop.last %},{% endif %}
{% endfor %}
FROM my_table;

Important Notes

Remember to handle commas or other separators carefully inside loops to avoid syntax errors.

You can loop over lists, ranges, or any iterable data in dbt Jinja templates.

Loops help keep your SQL DRY (Don't Repeat Yourself) and easier to maintain.

Summary

For loops let you write SQL that repeats patterns automatically.

Use {% for %} and {{ }} to insert dynamic parts.

Loops save time and reduce errors when working with many similar SQL pieces.

Practice

1. What is the main purpose of using {% for %} loops in dbt SQL models?

easy

A. To create permanent tables in the database

B. To stop SQL queries from running

C. To write Python code inside SQL models

D. To repeat SQL code patterns dynamically and reduce manual repetition

Solution

1. Step 1: Understand the role of {% for %} in dbt

   The {% for %} loop is used to repeat code blocks dynamically in dbt SQL models.

2. Step 2: Identify the benefit of repeating code

   Repeating code patterns saves time and reduces errors by avoiding manual copy-pasting.

3. Final Answer:

   To repeat SQL code patterns dynamically and reduce manual repetition -> Option D

4. Quick Check:

   For loops = dynamic repetition [OK]

Hint: For loops repeat code blocks automatically in dbt [OK]

Common Mistakes:

• Thinking for loops create tables
• Confusing dbt loops with Python loops
• Believing loops stop queries

2. Which of the following is the correct syntax to start a for loop in a dbt SQL model?

easy

A. {% loop item in list %}

B. {{ for item in list }}

C. {% for item in list %}

D. <% for item in list %>

Solution

1. Step 1: Recall dbt Jinja syntax for loops

   dbt uses Jinja templating where loops start with {% for variable in sequence %}.

2. Step 2: Compare options to correct syntax

   Only {% for item in list %} matches the correct Jinja for loop syntax.

3. Final Answer:

   {% for item in list %} -> Option C

4. Quick Check:

   dbt for loop syntax = {% for %} [OK]

Hint: dbt loops start with {% for variable in sequence %} [OK]

Common Mistakes:

• Using {{ }} instead of {% %} for loops
• Using wrong keywords like 'loop'
• Using angle brackets instead of curly braces

3. Given the following dbt code snippet, what will be the output SQL?

{% for col in ['id', 'name', 'age'] %}SELECT {{ col }} FROM users{% if not loop.last %} UNION ALL {% endif %}{% endfor %}

medium

A. SELECT id FROM users UNION ALL SELECT name FROM users UNION ALL SELECT age FROM users

B. SELECT id, name, age FROM users

C. SELECT id FROM users SELECT name FROM users SELECT age FROM users

D. SyntaxError due to missing commas

Solution

1. Step 1: Understand the loop iteration

   The loop runs over ['id', 'name', 'age'], producing three SELECT statements.

2. Step 2: Check the UNION ALL insertion

   Between each SELECT except the last, 'UNION ALL' is added to combine results.

3. Final Answer:

   SELECT id FROM users UNION ALL SELECT name FROM users UNION ALL SELECT age FROM users -> Option A

4. Quick Check:

   Loop + UNION ALL = combined selects [OK]

Hint: Look for loop.last to add UNION ALL except last [OK]

Common Mistakes:

• Ignoring the UNION ALL between selects
• Assuming all columns selected in one query
• Thinking commas separate selects

4. Identify the error in this dbt for loop snippet:

{% for col in columns %}SELECT {{ col }} FROM users{% endfor %}

Assuming columns = ['id', 'name'].

medium

A. Wrong loop syntax with {{ }} instead of {% %}

B. Missing UNION ALL or separator between SELECT statements

C. Incorrect variable name inside the loop

D. Loop variable should be capitalized

Solution

1. Step 1: Analyze the loop output

   The loop outputs two SELECT statements back to back without any separator.

2. Step 2: Identify missing SQL syntax

   Without UNION ALL or a separator, the SQL is invalid and will cause errors.

3. Final Answer:

   Missing UNION ALL or separator between SELECT statements -> Option B

4. Quick Check:

   Multiple SELECTs need UNION ALL [OK]

Hint: Add UNION ALL between selects in loops [OK]

Common Mistakes:

• Forgetting to add UNION ALL
• Using wrong loop syntax
• Misnaming variables inside loop

5. You want to create a dynamic SQL query in dbt that selects multiple columns from a table, but only if the column name is not empty or None. Which for loop code snippet correctly achieves this?

hard

A. {% for col in columns if col %}SELECT {{ col }} FROM users{% if not loop.last %} UNION ALL {% endif %}{% endfor %}

B. {% for col in columns %}SELECT {{ col }} FROM users{% if col != '' or col != None %} UNION ALL {% endif %}{% endfor %}

C. {% for col in columns %}{% if col %}SELECT {{ col }} FROM users UNION ALL {% endif %}{% endfor %}

D. {% for col in columns %}SELECT {{ col }} FROM users UNION ALL {% endfor %}

Solution

1. Step 1: Filter columns inside the for loop

   {% for col in columns if col %}SELECT {{ col }} FROM users{% if not loop.last %} UNION ALL {% endif %}{% endfor %} uses for col in columns if col to skip empty or None values correctly.

2. Step 2: Add UNION ALL only between valid selects

   {% for col in columns if col %}SELECT {{ col }} FROM users{% if not loop.last %} UNION ALL {% endif %}{% endfor %} adds UNION ALL only if not the last valid column, avoiding trailing UNION ALL.

3. Final Answer:

   {% for col in columns if col %}SELECT {{ col }} FROM users{% if not loop.last %} UNION ALL {% endif %}{% endfor %} -> Option A

4. Quick Check:

   Filter in loop + conditional UNION ALL = correct [OK]

Hint: Use 'if col' in loop to skip empty values [OK]

Common Mistakes:

• Using incorrect if condition syntax
• Adding UNION ALL after last select
• Not filtering out empty or None columns