RETURN and RETURN NEXT in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the RETURN statement do in a PostgreSQL function?

The RETURN statement immediately ends the function and sends back a single value or result to the caller.

beginner

What is the purpose of RETURN NEXT in PostgreSQL functions?

RETURN NEXT adds a row to the result set of a set-returning function without ending the function, allowing multiple rows to be returned one by one.

intermediate

How do RETURN and RETURN NEXT differ in their effect inside a PostgreSQL function?

RETURN ends the function and returns a single result immediately, while RETURN NEXT adds a row to the output and lets the function continue to add more rows.

beginner

In what kind of PostgreSQL function would you use RETURN NEXT?

You use RETURN NEXT in functions declared to return SETOF some type, meaning they return multiple rows instead of just one.

intermediate

What happens if you use RETURN inside a set-returning function before RETURN NEXT?

Using RETURN will immediately stop the function and return the current result, so any rows after RETURN NEXT will not be returned.

Which statement immediately ends a PostgreSQL function and returns a value?

A YIELD

B RETURN NEXT

C RETURN

D CONTINUE

RETURN immediately ends the function and returns a value to the caller.

What does RETURN NEXT do in a PostgreSQL function?

A Adds a row to the result set and continues the function

B Ends the function and returns a single value

C Skips the current iteration

D Resets the function state

RETURN NEXT adds a row to the output and lets the function continue to add more rows.

In which type of function is RETURN NEXT typically used?

A Functions returning a single scalar value

B Functions returning SETOF rows

C Trigger functions

D Aggregate functions

RETURN NEXT is used in functions that return SETOF rows, meaning multiple rows.

What happens if RETURN is used before RETURN NEXT in a set-returning function?

A The function stops and returns immediately

B The function continues normally

C An error is thrown

D RETURN NEXT is ignored but function continues

RETURN stops the function immediately, so any following RETURN NEXT statements are not executed.

Which statement would you use to return multiple rows one by one in a PostgreSQL function?

A RETURN

B RAISE

C EXECUTE

D RETURN NEXT

RETURN NEXT is used to return multiple rows one at a time in set-returning functions.

Explain the difference between RETURN and RETURN NEXT in PostgreSQL functions.

Think about when the function stops and how many rows are returned.

Describe a scenario where you would use RETURN NEXT in a PostgreSQL function.

Consider functions that return lists or sets of data.

Practice

1. What does the RETURN statement do in a PostgreSQL function?

easy

A. It loops through all rows in a table.

B. It adds a row to the output but keeps the function running.

C. It sends back one result and stops the function.

D. It creates a new table in the database.

Solution

1. Step 1: Understand the purpose of RETURN

   The RETURN statement immediately sends a single result back to the caller and ends the function execution.

2. Step 2: Compare with RETURN NEXT

   Unlike RETURN NEXT, which adds rows and continues, RETURN stops the function after sending one result.

3. Final Answer:

   It sends back one result and stops the function. -> Option C

4. Quick Check:

   RETURN = sends one result and stops [OK]

Hint: RETURN sends one result and stops function immediately [OK]

Common Mistakes:

• Confusing RETURN with RETURN NEXT
• Thinking RETURN returns multiple rows
• Assuming RETURN continues function execution

2. Which of the following is the correct syntax to return multiple rows from a PostgreSQL function?

easy

A. Use RETURN NEXT inside a loop to add each row to the output.

B. Use RETURN ROW to return multiple rows.

C. Use RETURN ALL to return all rows at once.

D. Use RETURN inside a loop to return each row.

Solution

1. Step 1: Identify how to return multiple rows

   RETURN NEXT is used inside loops to add each row to the output without stopping the function.

2. Step 2: Check syntax options

   RETURN alone stops the function after one row; RETURN ALL and RETURN ROW are not valid PostgreSQL syntax.

3. Final Answer:

   Use RETURN NEXT inside a loop to add each row to the output. -> Option A

4. Quick Check:

   RETURN NEXT = add rows, keep running [OK]

Hint: Use RETURN NEXT in loops to return multiple rows [OK]

Common Mistakes:

• Using RETURN instead of RETURN NEXT for multiple rows
• Assuming RETURN ALL or RETURN ROW exist
• Not placing RETURN NEXT inside a loop

3. Consider this PostgreSQL function snippet:

FOR i IN 1..3 LOOP
  RETURN NEXT i;
END LOOP;
RETURN;

What will be the output when this function is called?

medium

A. Syntax error due to RETURN NEXT usage

B. [3]

C. No output, function ends without returning

D. [1, 2, 3]

Solution

1. Step 1: Analyze the loop with RETURN NEXT

   The loop runs from 1 to 3, and each iteration adds the current number to the output using RETURN NEXT.

2. Step 2: Understand the final RETURN

   The final RETURN ends the function after all rows have been added, so the output is all numbers collected.

3. Final Answer:

   [1, 2, 3] -> Option D

4. Quick Check:

   RETURN NEXT adds rows; final RETURN stops function [OK]

Hint: RETURN NEXT inside loop collects rows; final RETURN stops [OK]

Common Mistakes:

• Thinking only last value is returned
• Confusing RETURN NEXT with RETURN
• Expecting syntax error from RETURN NEXT

4. You wrote this function:

CREATE FUNCTION test_func() RETURNS SETOF integer AS $$
DECLARE
  i integer := 1;
BEGIN
  RETURN i;
  RETURN NEXT i + 1;
END;
$$ LANGUAGE plpgsql;

What is the problem with this function?

medium

A. RETURN NEXT is used after RETURN, so it never executes.

B. RETURN cannot be used in functions returning SETOF.

C. Variable i is not initialized properly.

D. Function lacks a LOOP to return multiple rows.

Solution

1. Step 1: Check order of RETURN and RETURN NEXT

   RETURN immediately ends the function, so RETURN NEXT after it never runs.

2. Step 2: Understand function behavior

   Because RETURN is first, only one row is returned and the rest is ignored.

3. Final Answer:

   RETURN NEXT is used after RETURN, so it never executes. -> Option A

4. Quick Check:

   RETURN stops function; code after it is skipped [OK]

Hint: RETURN stops function; code after it won't run [OK]

Common Mistakes:

• Assuming RETURN NEXT runs after RETURN
• Thinking RETURN can't be used in SETOF functions
• Believing variable initialization causes error

5. You want to create a function that returns all even numbers from 1 to 10. Which of these function bodies correctly uses RETURN NEXT and RETURN to achieve this?

hard

A. FOR i IN 1..10 LOOP
IF i % 2 = 0 THEN
RETURN i;
END IF;
END LOOP;

B. FOR i IN 1..10 LOOP
IF i % 2 = 0 THEN
RETURN NEXT i;
END IF;
END LOOP;
RETURN;

C. FOR i IN 1..10 LOOP
RETURN NEXT i;
END LOOP;
RETURN;

D. RETURN NEXT 2;
RETURN 4;
RETURN NEXT 6;
RETURN NEXT 8;
RETURN NEXT 10;
RETURN;

Solution

1. Step 1: Identify correct use of RETURN NEXT in loop with condition

   FOR i IN 1..10 LOOP
     IF i % 2 = 0 THEN
       RETURN NEXT i;
     END IF;
   END LOOP;
   RETURN;

   loops 1 to 10, adds only even numbers with RETURN NEXT, then ends with RETURN.

2. Step 2: Check other options for errors

   FOR i IN 1..10 LOOP
     IF i % 2 = 0 THEN
       RETURN i;
     END IF;
   END LOOP;

   uses RETURN inside loop, stopping after first even number.

   FOR i IN 1..10 LOOP
     RETURN NEXT i;
   END LOOP;
   RETURN;

   returns all numbers, not just even.

   RETURN NEXT 2;
   RETURN 4;
   RETURN NEXT 6;
   RETURN NEXT 8;
   RETURN NEXT 10;
   RETURN;

   mixes RETURN NEXT and RETURN; RETURN 4 adds 4 and stops, returning only [2,4].

3. Final Answer:

   FOR i IN 1..10 LOOP IF i % 2 = 0 THEN RETURN NEXT i; END IF; END LOOP; RETURN; -> Option B

4. Quick Check:

   RETURN NEXT adds rows conditionally; RETURN ends function [OK]

Hint: Use RETURN NEXT inside loop with condition; end with RETURN [OK]

Common Mistakes:

• Using RETURN inside loop stopping early
• Returning all numbers instead of filtering
• Mixing RETURN and RETURN NEXT causing early termination