DO blocks for anonymous code in PostgreSQL

Introduction

DO blocks let you run small pieces of code inside the database without creating a permanent function.

You want to test a quick piece of code without saving it.

You need to perform a one-time task like updating data or logging.

You want to run procedural logic directly in the database.

You want to execute code that does not return data but performs actions.

You want to automate small maintenance tasks inside the database.

Syntax

PostgreSQL

DO $$
BEGIN
  -- your code here
END;
$$ LANGUAGE plpgsql;

The code inside DO runs immediately and is anonymous (no name).

Use LANGUAGE plpgsql to write procedural code in PostgreSQL.

Examples

This prints a simple message to the console.

PostgreSQL

DO $$
BEGIN
  RAISE NOTICE 'Hello, world!';
END;
$$ LANGUAGE plpgsql;

This increases salary by 10% for all employees in Sales.

PostgreSQL

DO $$
BEGIN
  UPDATE employees SET salary = salary * 1.1 WHERE department = 'Sales';
END;
$$ LANGUAGE plpgsql;

This counts employees and prints the total.

PostgreSQL

DO $$
DECLARE
  count_employees INTEGER;
BEGIN
  SELECT COUNT(*) INTO count_employees FROM employees;
  RAISE NOTICE 'Total employees: %', count_employees;
END;
$$ LANGUAGE plpgsql;

Sample Program

This block prints messages before and after a 1-second pause.

PostgreSQL

DO $$
BEGIN
  RAISE NOTICE 'Starting anonymous block';
  PERFORM pg_sleep(1); -- wait for 1 second
  RAISE NOTICE 'Finished anonymous block';
END;
$$ LANGUAGE plpgsql;

Important Notes

DO blocks do not return query results; they are for running code only.

Use RAISE NOTICE to print messages for debugging or information.

You can declare variables inside DO blocks using DECLARE.

Summary

DO blocks run anonymous procedural code immediately in PostgreSQL.

They are useful for quick tasks without creating permanent functions.

Use LANGUAGE plpgsql and write code inside BEGIN...END.

Practice

1. What is the main purpose of a DO block in PostgreSQL?

easy

A. To define a new permanent function

B. To create a new table in the database

C. To execute a SELECT query and return results

D. To run anonymous procedural code immediately without creating a permanent function

Solution

1. Step 1: Understand the role of DO blocks

   DO blocks allow running procedural code immediately without saving it as a function.

2. Step 2: Compare with other options

   Creating tables or functions is done with other commands, and DO blocks do not return query results.

3. Final Answer:

   To run anonymous procedural code immediately without creating a permanent function -> Option D

4. Quick Check:

   DO blocks = anonymous immediate code execution [OK]

Hint: DO blocks run code immediately without saving functions [OK]

Common Mistakes:

• Thinking DO blocks create permanent functions
• Confusing DO blocks with SELECT queries
• Assuming DO blocks create tables

2. Which of the following is the correct syntax to start a DO block in PostgreSQL?

easy

A. DO $$ BEGIN END $$ LANGUAGE plpgsql;

B. DO LANGUAGE plpgsql BEGIN END;

C. DO BEGIN $$ END LANGUAGE plpgsql;

D. DO $$ LANGUAGE plpgsql BEGIN END $$;

Solution

1. Step 1: Recall the correct DO block syntax

   The DO block uses dollar quoting $$ to enclose the code, with LANGUAGE plpgsql specified after the block.

2. Step 2: Check each option

   DO $$ BEGIN END $$ LANGUAGE plpgsql; correctly places $$ around BEGIN...END and specifies LANGUAGE plpgsql after the block.

3. Final Answer:

   DO $$ BEGIN END $$ LANGUAGE plpgsql; -> Option A

4. Quick Check:

   DO block syntax = DO $$ code $$ LANGUAGE plpgsql; [OK]

Hint: Use DO $$ ... $$ LANGUAGE plpgsql; to start DO blocks [OK]

Common Mistakes:

• Placing LANGUAGE plpgsql before BEGIN
• Not using dollar quoting $$
• Misordering keywords in the DO block

3. What will be the output of this DO block?

DO $$
BEGIN
  RAISE NOTICE 'Hello, PostgreSQL!';
END
$$ LANGUAGE plpgsql;

medium

A. It prints 'Hello, PostgreSQL!' as a notice message

B. It returns a result set with 'Hello, PostgreSQL!'

C. It causes a syntax error

D. It creates a permanent function named 'Hello, PostgreSQL!'

Solution

1. Step 1: Understand RAISE NOTICE in DO blocks

   RAISE NOTICE outputs a message to the client as an informational notice, not a query result.

2. Step 2: Analyze the DO block behavior

   The block runs immediately and prints the notice message but does not return rows or create functions.

3. Final Answer:

   It prints 'Hello, PostgreSQL!' as a notice message -> Option A

4. Quick Check:

   RAISE NOTICE outputs messages, not query results [OK]

Hint: RAISE NOTICE shows messages, no query output [OK]

Common Mistakes:

• Expecting query result rows
• Thinking it creates a function
• Confusing notice with error

4. Identify the error in this DO block:

DO $$
BEGIN
  PERFORM 1/0;
END
$$ LANGUAGE plpgsql;

medium

A. Syntax error due to missing semicolon

B. Division by zero runtime error

C. Missing LANGUAGE specification

D. Invalid use of PERFORM keyword

Solution

1. Step 1: Analyze the code inside DO block

   The statement PERFORM 1/0 attempts to divide 1 by zero, which is not allowed.

2. Step 2: Identify the error type

   This causes a runtime error (division by zero), not a syntax error or missing keyword.

3. Final Answer:

   Division by zero runtime error -> Option B

4. Quick Check:

   1/0 causes runtime error, not syntax [OK]

Hint: Check for runtime errors like division by zero [OK]

Common Mistakes:

• Confusing runtime error with syntax error
• Ignoring division by zero possibility
• Assuming PERFORM is invalid here

5. You want to update a table users to set active = false for all users who haven't logged in for over a year. Which DO block correctly performs this task?

hard

A. DO $$ BEGIN UPDATE users SET active = false WHERE last_login > NOW() - INTERVAL '1 year'; END $$ LANGUAGE plpgsql;

B. DO $$ BEGIN SELECT * FROM users WHERE last_login < NOW() - INTERVAL '1 year'; END $$ LANGUAGE plpgsql;

C. DO $$ BEGIN UPDATE users SET active = false WHERE last_login < NOW() - INTERVAL '1 year'; END $$ LANGUAGE plpgsql;

D. DO $$ UPDATE users SET active = false WHERE last_login < NOW() - INTERVAL '1 year'; END $$ LANGUAGE plpgsql;

Solution

1. Step 1: Check the DO block structure and logic

   DO $$ BEGIN UPDATE users SET active = false WHERE last_login < NOW() - INTERVAL '1 year'; END $$ LANGUAGE plpgsql; correctly uses BEGIN...END with an UPDATE statement and the right condition for last_login older than 1 year.

2. Step 2: Verify other options

   DO $$ BEGIN SELECT * FROM users WHERE last_login < NOW() - INTERVAL '1 year'; END $$ LANGUAGE plpgsql; only selects rows, no update. DO $$ UPDATE users SET active = false WHERE last_login < NOW() - INTERVAL '1 year'; END $$ LANGUAGE plpgsql; misses BEGIN...END block. DO $$ BEGIN UPDATE users SET active = false WHERE last_login > NOW() - INTERVAL '1 year'; END $$ LANGUAGE plpgsql; uses wrong condition (greater than instead of less than).

3. Final Answer:

   DO $$ BEGIN UPDATE users SET active = false WHERE last_login < NOW() - INTERVAL '1 year'; END $$ LANGUAGE plpgsql; -> Option C

4. Quick Check:

   Correct DO block with UPDATE and condition [OK]

Hint: Use BEGIN...END with UPDATE and correct WHERE condition [OK]

Common Mistakes:

• Omitting BEGIN...END block
• Using wrong comparison operator in WHERE
• Using SELECT instead of UPDATE