Variables and control flow in dbt - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a variable in dbt and how is it used?

A variable in dbt is a named value that you can set and reuse in your models or macros. It helps make your code flexible by allowing you to change values without editing the whole code.

beginner

How do you define and use an if-else control flow in dbt?

In dbt, you use Jinja templating for control flow. An if-else looks like: {% if condition %} do something {% else %} do something else {% endif %}. It helps run different code based on conditions.

beginner

Explain the purpose of the 'set' statement in dbt.

The 'set' statement assigns a value to a variable inside a dbt model or macro. For example, {% set my_var = 10 %} stores 10 in my_var for later use in the code.

intermediate

What is the difference between 'if' and 'elif' in dbt control flow?

'if' starts a condition check. 'elif' (else if) checks another condition only if the previous 'if' was false. It helps check multiple conditions in order.

intermediate

How can you use variables to make dbt models more dynamic?

You can pass variables to dbt models or macros to change behavior without changing code. For example, use a variable to filter data by date or region dynamically.

Which syntax correctly sets a variable named 'threshold' to 100 in dbt?

A {% set threshold = 100 %}

B threshold = 100

C {{ set threshold = 100 }}

D let threshold = 100

In dbt, variables are set using Jinja syntax with {% set variable = value %}.

What does the following dbt code do? {% if is_active %} SELECT * FROM users {% else %} SELECT * FROM guests {% endif %}

A Always selects from users table

B Selects from users if is_active is true, else from guests

C Selects from guests table only

D Throws an error because of syntax

The if-else block chooses the table based on the value of is_active.

How do you check multiple conditions in dbt control flow?

A Using if, elif, and else blocks

B Using multiple if statements without else

C Using switch-case statements

D Using only else statements

dbt uses Jinja which supports if, elif, and else for multiple condition checks.

Which of these is NOT a valid way to use variables in dbt?

A Passing variables to macros

B Using variables to filter data

C Changing variable values during model execution

D Setting variables with {% set %} statement

Variables in dbt are set once per execution and cannot be changed dynamically during model run.

What happens if the condition in an if statement is false and there is no else block?

A Code inside if runs twice

B Code inside if runs anyway

C Error is thrown

D Code inside if is skipped

If the condition is false and no else block exists, the code inside if is skipped.

Explain how variables and control flow work together in dbt to make models flexible.

Think about how you can change model behavior without rewriting code.

Describe a real-life example where you would use variables and if-else in a dbt model.

Imagine you want to run different queries based on user input or environment.

Practice

1. What does the {% set %} statement do in dbt?

easy

A. It deletes a table from the database.

B. It runs a SQL query directly.

C. It defines a new model in dbt.

D. It creates a variable to store a value for reuse.

Solution

1. Step 1: Understand the purpose of {% set %}

   The {% set %} statement in dbt is used to create a variable that holds a value.

2. Step 2: Identify its use in code reuse

   This variable can then be reused multiple times in your dbt project, making code cleaner and easier to maintain.

3. Final Answer:

   It creates a variable to store a value for reuse. -> Option D

4. Quick Check:

   {% set %} creates variables [OK]

Hint: Remember: {% set %} means 'store this value' [OK]

Common Mistakes:

• Thinking {% set %} runs SQL directly
• Confusing it with model creation
• Assuming it deletes data

2. Which of the following is the correct syntax to create a variable named my_var with value 10 in dbt?

easy

A. {% set my_var = 10 %}

B. {% var my_var = 10 %}

C. {{ set my_var = 10 }}

D. {% set 'my_var' = 10 %}

Solution

1. Step 1: Recall dbt variable syntax

   In dbt, variables are created using the Jinja syntax: {% set variable_name = value %}.

2. Step 2: Check each option

   {% set my_var = 10 %} matches the correct syntax exactly. Options B and C use wrong keywords or delimiters. {% set 'my_var' = 10 %} incorrectly quotes the variable name.

3. Final Answer:

   {% set my_var = 10 %} -> Option A

4. Quick Check:

   Correct variable syntax uses {% set %} [OK]

Hint: Use {% set var = value %} without quotes around var [OK]

Common Mistakes:

• Using {{ }} instead of {% %} for setting variables
• Quoting variable names incorrectly
• Using wrong keywords like var

3. Given the following dbt code snippet, what will be the output of {{ greeting }}?

{% set hour = 15 %}
{% if hour < 12 %}
  {% set greeting = 'Good morning' %}
{% elif hour < 18 %}
  {% set greeting = 'Good afternoon' %}
{% else %}
  {% set greeting = 'Good evening' %}
{% endif %}
{{ greeting }}

medium

A. Good morning

B. Good evening

C. Good afternoon

D. No output (error)

Solution

1. Step 1: Evaluate the value of hour

   The variable hour is set to 15.

2. Step 2: Check the if-elif-else conditions

   Since 15 is not less than 12, the first condition fails. It is less than 18, so the second condition is true, setting greeting to 'Good afternoon'.

3. Final Answer:

   Good afternoon -> Option C

4. Quick Check:

   15 < 18 -> 'Good afternoon' [OK]

Hint: Check conditions in order; first true branch runs [OK]

Common Mistakes:

• Assuming hour 15 is morning
• Ignoring elif and jumping to else
• Thinking variable is undefined

4. Identify the error in this dbt snippet:

{% set total = 0 %}
{% for i in range(3) %}
  {% set total = total + i %}
{% endfor %}
{{ total }}

medium

A. The range function is not available in dbt Jinja.

B. Variables cannot be updated inside a loop.

C. The loop syntax is incorrect; missing parentheses.

D. The variable total should be declared with var.

Solution

1. Step 1: Understand Jinja in dbt limitations

   dbt uses Jinja templating, which does not support Python's range() function directly.

2. Step 2: Identify the error cause

   Using range(3) will cause an error because range is undefined in this context.

3. Final Answer:

   The range function is not available in dbt Jinja. -> Option A

4. Quick Check:

   No range() in dbt Jinja [OK]

Hint: Remember: Jinja loops use lists, not Python range() [OK]

Common Mistakes:

• Assuming Python functions work in Jinja
• Trying to update variables inside loops without workarounds
• Using wrong loop syntax

5. You want to create a dbt model that uses a variable env to switch between two schemas: dev_schema if env is 'dev', else prod_schema. Which code snippet correctly implements this control flow?

hard

A. {% set schema = 'dev_schema' if env == 'dev' else 'prod_schema' %}
SELECT * FROM {{ schema }}.my_table

B. {% if env == 'dev' %}
{% set schema = 'dev_schema' %}
{% else %}
{% set schema = 'prod_schema' %}
{% endif %}
SELECT * FROM {{ schema }}.my_table

C. {% set schema = env == 'dev' and 'dev_schema' or 'prod_schema' %}
SELECT * FROM {{ schema }}.my_table

D. {% set schema = 'prod_schema' %}
{% if env == 'dev' %}
{% set schema = 'dev_schema' %}
SELECT * FROM {{ schema }}.my_table

Solution

1. Step 1: Understand conditional variable assignment in dbt

   dbt supports control flow with {% if %} and {% else %} blocks to set variables based on conditions.

2. Step 2: Check each option for correctness

   {% if env == 'dev' %} {% set schema = 'dev_schema' %} {% else %} {% set schema = 'prod_schema' %} {% endif %} SELECT * FROM {{ schema }}.my_table uses proper if-else blocks to set schema correctly before using it in the query. {% set schema = 'dev_schema' if env == 'dev' else 'prod_schema' %} SELECT * FROM {{ schema }}.my_table uses Python-style inline if which is not supported in Jinja. {% set schema = env == 'dev' and 'dev_schema' or 'prod_schema' %} SELECT * FROM {{ schema }}.my_table uses a Python boolean expression which is not valid in Jinja. {% set schema = 'prod_schema' %} {% if env == 'dev' %} {% set schema = 'dev_schema' %} SELECT * FROM {{ schema }}.my_table sets schema first then tries to reset inside if but the query is inside the if block, causing scope issues.

3. Final Answer:

   Option B correctly uses if-else to set schema and then queries it. -> Option B

4. Quick Check:

   Use {% if %}...{% else %}...{% endif %} for control flow [OK]

Hint: Use full if-else blocks to set variables, not inline Python [OK]

Common Mistakes:

• Trying Python inline if syntax in Jinja
• Placing SQL inside if blocks incorrectly
• Not setting variable before using it