Query builder in NestJS - Step-by-Step Execution

Concept Flow - Query builder

Start Query Builder

↓

Initialize Query Object

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Add Select Fields

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Add Where Conditions

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Add Joins (optional)

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Add Order/Limit (optional)

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Execute Query

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Return Results

This flow shows how a query builder in NestJS creates a query step-by-step, adding parts like select fields, conditions, joins, and finally runs the query to get results.

Execution Sample

NestJS

const query = this.repo.createQueryBuilder('user')
  .select(['user.id', 'user.name'])
  .where('user.age > :age', { age: 18 })
  .orderBy('user.name', 'ASC')
  .getMany();

Builds a query to select user id and name where age is over 18, orders by name ascending, and fetches the results.

Execution Table

Step	Action	Query State	Parameters	Output
1	Initialize QueryBuilder with alias 'user'	SELECT * FROM user	{}	QueryBuilder object created
2	Add select fields ['user.id', 'user.name']	SELECT user.id, user.name FROM user	{}	Select fields set
3	Add where condition 'user.age > :age' with {age:18}	SELECT user.id, user.name FROM user WHERE user.age > :age	{age:18}	Where condition added
4	Add order by 'user.name' ASC	SELECT user.id, user.name FROM user WHERE user.age > :age ORDER BY user.name ASC	{age:18}	Order by added
5	Execute getMany()	Final SQL executed	{age:18}	Returns array of user objects matching criteria
6	End	Query complete	{}	Results returned

💡 Query executed and results returned, no more steps.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
query	undefined	QueryBuilder with select fields	QueryBuilder with where condition	QueryBuilder with order by	Promise of results	Array of user objects

Key Moments - 2 Insights

Why do we use ':age' in the where condition instead of directly writing the number 18?

Using ':age' is a parameter placeholder that safely inserts the value 18, preventing SQL injection and allowing reuse. See execution_table step 3 where parameters {age:18} are passed separately.

What happens if we forget to call getMany() at the end?

Without getMany(), the query is not executed and no results are returned. The query builder only builds the query until getMany() triggers execution (see execution_table step 5).

Visual Quiz - 3 Questions

Look at the execution table, what is the query state after step 3?

A SELECT * FROM user

B SELECT user.id, user.name FROM user WHERE user.age > :age

C SELECT user.id, user.name FROM user ORDER BY user.name ASC

D Final SQL executed

💡 Hint

Check the 'Query State' column in row with Step 3.

At which step is the query actually executed to get results?

A Step 2

B Step 3

C Step 5

D Step 4

💡 Hint

Look for 'Execute getMany()' action in the execution_table.

If we change the where condition to 'user.age > :age' with {age:21}, how does the parameters column change at step 3?

A {age:21}

B {}

C {age:18}

D No change

💡 Hint

Parameters column shows the values passed for placeholders, see step 3.

Concept Snapshot

NestJS Query Builder:
- Start with createQueryBuilder(alias)
- Chain select(), where(), orderBy(), join() as needed
- Use parameter placeholders like ':param' with values
- Call getMany() or getOne() to execute
- Returns results as objects

Build queries step-by-step, then run to get data.

Full Transcript

This visual execution shows how NestJS Query Builder works step-by-step. First, it creates a query builder object with an alias. Then it adds select fields to choose which columns to fetch. Next, it adds a where condition using a parameter placeholder to safely insert values. After that, it adds an order by clause to sort results. Finally, calling getMany() executes the SQL query and returns matching user objects. Variables track the query state and parameters at each step. Key moments explain why parameters are used and the importance of calling getMany(). The quiz tests understanding of query state, execution step, and parameter changes.