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# PostgreSQL BEFORE INSERT Trigger

**Summary**: in this tutorial, you will learn how to create a PostgreSQL `BEFORE INSERT` trigger associated with a table.

## Introduction to PostgreSQL BEFORE INSERT trigger

A trigger is a database object that automatically calls a function when an event such as `INSERT`, `UPDATE`, and `DELETE` statement occurs on the associated table.

A `BEFORE INSERT` trigger is activated before an `INSERT` event occurs on a table. To create a `BEFORE INSERT` trigger, you follow these steps:

First, define a trigger function that will execute before the `INSERT` event occurs:

```plsql
CREATE OR REPLACE FUNCTION trigger_function()
   RETURNS TRIGGER
   LANGUAGE PLPGSQL
AS
$$
BEGIN
   -- trigger logic
   -- ...
   RETURN NEW;
END;
$$
```

At the end of the function, you need to place the `RETURN NEW` statement

Second, create a `BEFORE INSERT` trigger and associate a trigger function with it:

```sql
CREATE TRIGGER trigger_name
BEFORE INSERT
ON table_name
FOR EACH {ROW | STATEMENT}
EXECUTE FUNCTION trigger_function();
```

## PostgreSQL BEFORE INSERT trigger example

First, create a table called `inventory` to store inventory data:

```sql
CREATE TABLE inventory(
    product_id INT PRIMARY KEY,
    quantity INT NOT NULL DEFAULT 0
);
```

Second, create a table called `inventory_stat` that stores the total quantity of all products:

```sql
CREATE TABLE inventory_stat(
    total_qty INT
);
```

Third, define a function that increases the total quantity in the `inventory_stat` before a row is inserted into the `inventory` table:

```plsql
CREATE OR REPLACE FUNCTION update_total_qty()
RETURNS TRIGGER
LANGUAGE PLPGSQL
AS
$$
DECLARE
   p_row_count INT;
BEGIN
   SELECT COUNT(*) FROM inventory_stat
   INTO p_row_count;

   IF p_row_count > 0 THEN
      UPDATE inventory_stat
      SET total_qty = total_qty + NEW.quantity;
   ELSE
      INSERT INTO inventory_stat(total_qty)
      VALUES(new.quantity);
   END IF;
   RETURN NEW;
END;
$$;
```

If the inventory_stat table has no rows, the function inserts a new row with the quantity being inserted into the inventory table. Otherwise, it updates the existing quantity.

Fourth, define a `BEFORE INSERT` trigger associated with the `inventory` table:

```sql
CREATE TRIGGER inventory_before_insert
BEFORE INSERT
ON inventory
FOR EACH ROW
EXECUTE FUNCTION update_total_qty();
```

Fifth, insert a row into the inventory table:

```sql
INSERT INTO inventory(product_id, quantity)
VALUES(1, 100)
RETURNING *;
```

Output:

```text
 product_id | quantity
------------+----------
          1 |      100
(1 row)
```

Sixth, retrieve data from the `inventory_stat` table:

```sql
SELECT * FROM inventory_stat;
```

Output:

```text
 total_qty
-----------
       100
(1 row)
```

Seventh, insert another row into the `inventory` table:

```sql
INSERT INTO inventory(product_id, quantity)
VALUES(2, 200)
RETURNING *;
```

Output:

```text
 product_id | quantity
------------+----------
          2 |      200
(1 row)
```

Eighth, retrieve the data from the `inventory_stat` table:

```text
 total_qty
-----------
       300
(1 row)
```

## Summary

- A `BEFORE INSERT` trigger is activated before an `INSERT` event occurs on a table.

---

## Related docs (PostgreSQL Triggers)

- [Introduction](https://neon.com/postgresql/postgresql-triggers/introduction-postgresql-trigger)
- [CREATE TRIGGER](https://neon.com/postgresql/postgresql-triggers/creating-first-trigger-postgresql)
- [DROP TRIGGER](https://neon.com/postgresql/postgresql-triggers/postgresql-drop-trigger)
- [ALTER TRIGGER](https://neon.com/postgresql/postgresql-triggers/postgresql-alter-trigger)
- [AFTER INSERT Trigger](https://neon.com/postgresql/postgresql-triggers/postgresql-after-insert-trigger)
- [BEFORE UPDATE Trigger](https://neon.com/postgresql/postgresql-triggers/postgresql-before-update-trigger)
- [AFTER UPDATE Trigger](https://neon.com/postgresql/postgresql-triggers/postgresql-after-update-trigger)
- [BEFORE DELETE Trigger](https://neon.com/postgresql/postgresql-triggers/postgresql-before-delete-trigger)
- [AFTER DELETE Trigger](https://neon.com/postgresql/postgresql-triggers/postgresql-after-delete-trigger)
- [INSTEAD OF Triggers](https://neon.com/postgresql/postgresql-triggers/postgresql-instead-of-triggers)
- [BEFORE TRUNCATE Trigger](https://neon.com/postgresql/postgresql-triggers/postgresql-before-truncate-trigger)
- [Disable Triggers](https://neon.com/postgresql/postgresql-triggers/managing-postgresql-trigger)
- [Enable Triggers](https://neon.com/postgresql/postgresql-triggers/enable-triggers)
- [List All Triggers](https://neon.com/postgresql/postgresql-triggers/how-to-list-all-triggers-in-postgresql)
- [Event Triggers](https://neon.com/postgresql/postgresql-triggers/postgresql-event-trigger)
- [Conditional Triggers](https://neon.com/postgresql/postgresql-triggers/postgresql-trigger-when-condition)