Variables and control flow in dbt - Step-by-Step Execution

Concept Flow - Variables and control flow

Start

↓

Set Variable

↓

Check Condition

↓

Execute Block

↓

Update Variable or End

↩Back to Check or End

This flow shows how dbt sets variables, checks conditions, executes code blocks based on conditions, and updates variables or ends.

Execution Sample

dbt

{% set my_var = 0 %}

{% if my_var < 2 %}
  {{ log('Variable is less than 2') }}
{% endif %}

This dbt snippet sets a variable and uses an if condition to log a message if the variable is less than 2.

Execution Table

Step	Action	Variable 'my_var'	Condition 'my_var < 2'	Branch Taken	Output
1	Initialize variable 'my_var'	0	N/A	N/A	Variable set to 0
2	Evaluate condition 'my_var < 2'	0	True	Yes	Log message: 'Variable is less than 2'
3	End of control flow	0	N/A	N/A	Execution stops

💡 Reached end of code after condition check and logging

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
my_var	undefined	0	0	0

Key Moments - 2 Insights

Why does the condition 'my_var < 2' evaluate to True when 'my_var' is 0?

Because 0 is less than 2, the condition is True as shown in execution_table row 2, so the code inside the if block runs.

What happens if the variable 'my_var' was 3 instead of 0?

The condition 'my_var < 2' would be False, so the branch taken would be No, skipping the log message. This is shown by the condition check logic in execution_table.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'my_var' at Step 2?

A 2

B 0

C 1

D undefined

💡 Hint

Check the 'Variable my_var' column at Step 2 in the execution_table.

At which step does the condition 'my_var < 2' get evaluated?

A Step 2

B Step 1

C Step 3

D No step

💡 Hint

Look at the 'Condition' column in the execution_table to find when it is evaluated.

If 'my_var' was set to 3, what would happen at Step 2?

A Condition True, log message printed

B Error occurs

C Condition False, log message skipped

D Variable changes to 0

💡 Hint

Refer to key_moments explanation about variable values and condition outcomes.

Concept Snapshot

dbt variables store values used in control flow.
Use {% if name < value %} to check conditions.
Code inside if runs only if condition is True.
Variables do not change unless explicitly updated.
Control flow ends after last statement.

Full Transcript

In dbt, variables hold values that control the flow of code. We start by setting a variable, like 'my_var' to 0. Then we check if 'my_var' is less than 2. Since 0 is less than 2, the condition is True, so the code inside the if block runs and logs a message. If the variable was 3, the condition would be False and the code inside the if block would be skipped. Variables keep their values unless changed. The flow ends after the last statement.

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