NEW and OLD record access in PostgreSQL

Introduction

NEW and OLD let you see data before and after a change in a table. This helps you check or use the changed data inside triggers.

When you want to log changes made to a table automatically.

When you need to check if a value changed before allowing an update.

When you want to copy old data to another table before it gets updated or deleted.

When you want to enforce rules based on how data changes in a table.

Syntax

PostgreSQL

CREATE TRIGGER trigger_name
BEFORE|AFTER INSERT|UPDATE|DELETE ON table_name
FOR EACH ROW
EXECUTE FUNCTION function_name();

-- Inside the trigger function:
-- Use NEW.column_name to access new data (for INSERT or UPDATE)
-- Use OLD.column_name to access old data (for UPDATE or DELETE)

NEW is available in INSERT and UPDATE triggers to see the new row data.

OLD is available in UPDATE and DELETE triggers to see the old row data.

Examples

This trigger runs after an employee record is updated.

PostgreSQL

CREATE TRIGGER log_update
AFTER UPDATE ON employees
FOR EACH ROW
EXECUTE FUNCTION log_employee_changes();

This function prints old and new salary values when an update happens.

PostgreSQL

CREATE OR REPLACE FUNCTION log_employee_changes() RETURNS trigger AS $$
BEGIN
  RAISE NOTICE 'Old salary: %, New salary: %', OLD.salary, NEW.salary;
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;

Sample Program

This example creates a table and a trigger that prints a message when an employee's salary changes.

PostgreSQL

CREATE TABLE employees (
  id SERIAL PRIMARY KEY,
  name TEXT,
  salary INT
);

CREATE OR REPLACE FUNCTION log_salary_change() RETURNS trigger AS $$
BEGIN
  RAISE NOTICE 'Salary changed from % to % for employee %', OLD.salary, NEW.salary, NEW.name;
  RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER salary_change_trigger
AFTER UPDATE ON employees
FOR EACH ROW
EXECUTE FUNCTION log_salary_change();

INSERT INTO employees (name, salary) VALUES ('Alice', 50000);
UPDATE employees SET salary = 55000 WHERE name = 'Alice';

Important Notes

Triggers using NEW and OLD must be row-level (FOR EACH ROW) to access individual rows.

In INSERT triggers, OLD is not available because there is no old row.

In DELETE triggers, NEW is not available because the row is being removed.

Summary

NEW and OLD let you access data before and after changes inside triggers.

Use NEW for new data in INSERT and UPDATE triggers.

Use OLD for old data in UPDATE and DELETE triggers.

Practice

1. In a PostgreSQL trigger function, which record variable would you use to access the new row data after an INSERT operation?

easy

A. PREVIOUS

B. NEW

C. CURRENT

D. OLD

Solution

1. Step 1: Understand trigger timing for INSERT

   For an INSERT operation, the new row is being added, so the trigger can access the new data using the NEW record.

2. Step 2: Identify correct record variable

   The OLD record is not available for INSERT because there is no previous row. Therefore, NEW is used to access the inserted row.

3. Final Answer:

   NEW -> Option B

4. Quick Check:

   INSERT uses NEW = D [OK]

Hint: Use NEW for inserted or updated rows, OLD for deleted or old rows [OK]

Common Mistakes:

• Using OLD in INSERT triggers
• Confusing NEW and OLD for UPDATE
• Assuming CURRENT or PREVIOUS exist

2. Which of the following is the correct syntax to access the old value of a column named price inside a DELETE trigger in PostgreSQL?

easy

A. OLD.price

B. NEW.price

C. OLD->price

D. NEW->price

Solution

1. Step 1: Identify record variable for DELETE

   In a DELETE trigger, the row is being removed, so the old data is accessible via OLD.

2. Step 2: Use correct syntax for column access

   PostgreSQL uses dot notation to access columns in record variables, so OLD.price is correct.

3. Final Answer:

   OLD.price -> Option A

4. Quick Check:

   DELETE uses OLD.column = A [OK]

Hint: Use dot notation with OLD for deleted row columns [OK]

Common Mistakes:

• Using NEW in DELETE triggers
• Using arrow (->) instead of dot for record access
• Confusing syntax for JSON operators

3. Consider this trigger function snippet for an UPDATE operation:

IF NEW.quantity < OLD.quantity THEN
  RAISE NOTICE 'Quantity decreased from % to %', OLD.quantity, NEW.quantity;
END IF;

What will be the output if the old quantity was 10 and the new quantity is 7?

medium

A. Quantity decreased from 10 to 7

B. Quantity decreased from 7 to 10

C. No output

D. Syntax error

Solution

1. Step 1: Understand the condition in the IF statement

   The condition checks if the new quantity is less than the old quantity. Here, 7 < 10 is true.

2. Step 2: Analyze the RAISE NOTICE output

   The message prints the old quantity first, then the new quantity, so it will output: 'Quantity decreased from 10 to 7'.

3. Final Answer:

   Quantity decreased from 10 to 7 -> Option A

4. Quick Check:

   NEW < OLD triggers notice = A [OK]

Hint: Compare NEW and OLD values carefully in UPDATE triggers [OK]

Common Mistakes:

• Mixing up NEW and OLD values in output
• Assuming no output when condition is true
• Confusing syntax of RAISE NOTICE

4. You wrote this trigger function for DELETE:

CREATE FUNCTION trg_delete_check() RETURNS trigger AS $$
BEGIN
  IF NEW.id IS NULL THEN
    RAISE EXCEPTION 'ID cannot be null';
  END IF;
  RETURN OLD;
END;
$$ LANGUAGE plpgsql;

What is the error in this function?

medium

A. RAISE EXCEPTION syntax is wrong

B. RETURN OLD is invalid in DELETE triggers

C. Using NEW in a DELETE trigger where only OLD is available

D. Function must return VOID, not trigger

Solution

1. Step 1: Check record variables in DELETE triggers

   In DELETE triggers, the NEW record is not available because no new row is inserted or updated.

2. Step 2: Identify incorrect usage of NEW

   The function incorrectly uses NEW.id, which causes an error. It should use OLD.id instead.

3. Final Answer:

   Using NEW in a DELETE trigger where only OLD is available -> Option C

4. Quick Check:

   DELETE triggers have OLD, not NEW = C [OK]

Hint: Use OLD in DELETE triggers; NEW is unavailable [OK]

Common Mistakes:

• Using NEW in DELETE triggers
• Returning OLD incorrectly
• Misunderstanding trigger return types

5. You want to create a trigger that logs changes to a salary column only when the salary is updated to a higher value. Which trigger condition and record access correctly implements this in PostgreSQL?

hard

A. IF OLD.salary > NEW.salary THEN INSERT INTO log_table VALUES (OLD.id, OLD.salary); END IF;

B. IF NEW.salary < OLD.salary THEN INSERT INTO log_table VALUES (NEW.id, NEW.salary); END IF;

C. IF NEW.salary = OLD.salary THEN INSERT INTO log_table VALUES (NEW.id, NEW.salary); END IF;

D. IF NEW.salary > OLD.salary THEN INSERT INTO log_table VALUES (NEW.id, NEW.salary); END IF;

Solution

1. Step 1: Understand the condition for logging

   The trigger should log only when the new salary is greater than the old salary, so the condition is NEW.salary > OLD.salary.

2. Step 2: Use correct record variables for UPDATE

   The new salary and id come from NEW because the row is updated with new values.

3. Final Answer:

   IF NEW.salary > OLD.salary THEN INSERT INTO log_table VALUES (NEW.id, NEW.salary); END IF; -> Option D

4. Quick Check:

   Log when NEW > OLD salary = B [OK]

Hint: Compare NEW and OLD to detect increases, then log NEW data [OK]

Common Mistakes:

• Reversing NEW and OLD in condition
• Logging when salary decreases
• Using equality instead of greater than