Cursor declaration and usage in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a cursor in PostgreSQL?

A cursor is a database object used to retrieve a few rows from a query result at a time. It allows you to process query results row by row instead of all at once.

beginner

How do you declare a cursor in PostgreSQL?

You declare a cursor using the syntax:
DECLARE cursor_name CURSOR FOR SELECT_statement;
This prepares the cursor to fetch rows from the SELECT query.

beginner

What command is used to fetch rows from a cursor?

The FETCH command is used to retrieve rows from a cursor. For example: FETCH NEXT FROM cursor_name; gets the next row.

intermediate

Why should you close a cursor after use?

Closing a cursor releases the memory and resources it uses. Use CLOSE cursor_name; to close it when done.

intermediate

Explain the typical steps to use a cursor in PostgreSQL.

1. Declare the cursor with a SELECT query.
2. Open the cursor (optional in PostgreSQL, DECLARE opens it).
3. Fetch rows one by one or in blocks.
4. Close the cursor to free resources.

Which SQL command declares a cursor in PostgreSQL?

A DECLARE cursor_name CURSOR FOR SELECT * FROM table;

B OPEN cursor_name;

C FETCH cursor_name;

D CLOSE cursor_name;

The DECLARE command is used to create a cursor with a query.

What does the FETCH command do?

A Retrieves rows from the cursor

B Closes the cursor

C Declares a new cursor

D Opens the cursor

FETCH retrieves rows from the cursor one at a time or in groups.

Why is it important to CLOSE a cursor?

A To start fetching rows

B To declare the cursor

C To release resources used by the cursor

D To open the cursor

Closing a cursor frees memory and resources it uses.

Which of these is NOT a typical step when using a cursor?

A Declare the cursor

B Insert rows into the cursor

C Fetch rows

D Close the cursor

You cannot insert rows into a cursor; it only reads query results.

In PostgreSQL, when is a cursor opened?

A When you fetch from it

B When you commit a transaction

C When you close it

D When you declare it

In PostgreSQL, DECLARE opens the cursor immediately.

Describe how to declare, fetch from, and close a cursor in PostgreSQL.

Think about the three main commands: DECLARE, FETCH, CLOSE.

Explain why cursors are useful when working with large query results.

Consider memory and performance when handling big data.

Practice

1. What is the primary purpose of declaring a cursor in PostgreSQL?

easy

A. To speed up bulk inserts

B. To process query results one row at a time

C. To create a new table in the database

D. To backup the database automatically

Solution

1. Step 1: Understand what a cursor does

   A cursor allows you to handle query results row by row instead of all at once.

2. Step 2: Compare options with cursor purpose

   Only To process query results one row at a time describes this behavior; others describe unrelated tasks.

3. Final Answer:

   To process query results one row at a time -> Option B

4. Quick Check:

   Cursor purpose = process rows one by one [OK]

Hint: Cursors handle rows stepwise, not bulk operations [OK]

Common Mistakes:

• Confusing cursors with table creation
• Thinking cursors speed up inserts
• Assuming cursors automate backups

2. Which of the following is the correct syntax to declare a cursor named cur_emp for selecting all rows from employees table?

easy

A. CREATE CURSOR cur_emp AS SELECT * FROM employees;

B. OPEN cur_emp CURSOR SELECT * FROM employees;

C. DECLARE cur_emp CURSOR FOR SELECT * FROM employees;

D. FETCH cur_emp FROM employees;

Solution

1. Step 1: Recall cursor declaration syntax

   In PostgreSQL, cursors are declared with DECLARE cursor_name CURSOR FOR query.

2. Step 2: Match syntax with options

   DECLARE cur_emp CURSOR FOR SELECT * FROM employees; matches this exactly; others use incorrect keywords or order.

3. Final Answer:

   DECLARE cur_emp CURSOR FOR SELECT * FROM employees; -> Option C

4. Quick Check:

   DECLARE + CURSOR + FOR + query = correct syntax [OK]

Hint: Use DECLARE ... CURSOR FOR ... to declare [OK]

Common Mistakes:

• Using OPEN instead of DECLARE for declaration
• Confusing FETCH with DECLARE
• Using CREATE CURSOR which is invalid syntax

3. Given the following code snippet, what will be the output after fetching from the cursor?

DECLARE cur_emp CURSOR FOR SELECT id FROM employees ORDER BY id LIMIT 3;
OPEN cur_emp;
FETCH NEXT FROM cur_emp;
FETCH NEXT FROM cur_emp;

medium

A. First two employee ids in ascending order

B. All employee ids from the table

C. Syntax error due to missing CLOSE statement

D. Empty result because cursor is not opened

Solution

1. Step 1: Understand cursor declaration and fetch

   The cursor selects 3 employee ids ordered ascending. FETCH NEXT retrieves one row each time.

2. Step 2: Analyze fetch calls

   Two FETCH NEXT calls return the first two rows from the cursor result.

3. Final Answer:

   First two employee ids in ascending order -> Option A

4. Quick Check:

   Two FETCH NEXT = two rows fetched [OK]

Hint: Each FETCH returns one row in cursor order [OK]

Common Mistakes:

• Assuming FETCH returns all rows at once
• Thinking missing CLOSE causes syntax error
• Believing cursor must be closed before fetching

4. Identify the error in the following cursor usage:

DECLARE cur_dept CURSOR FOR SELECT name FROM departments;
FETCH NEXT FROM cur_dept;
OPEN cur_dept;
CLOSE cur_dept;

medium

A. Cursor declaration syntax is incorrect

B. Cursor is declared after fetching

C. Cursor is closed before declaration

D. Cursor is fetched before it is opened

Solution

1. Step 1: Check the order of cursor operations

   Cursors must be declared, then opened, then fetched, then closed.

2. Step 2: Identify incorrect sequence

   Here, FETCH is called before OPEN, which is invalid.

3. Final Answer:

   Cursor is fetched before it is opened -> Option D

4. Quick Check:

   OPEN must precede FETCH [OK]

Hint: Always OPEN cursor before FETCH [OK]

Common Mistakes:

• Fetching before opening cursor
• Closing cursor before opening
• Misordering declaration and fetch

5. You want to process all rows from orders table one by one using a cursor in a PL/pgSQL function. Which sequence of statements correctly implements this?

hard

A. DECLARE cur_orders CURSOR FOR SELECT * FROM orders; OPEN cur_orders; LOOP FETCH cur_orders INTO rec; EXIT WHEN NOT FOUND; -- process rec END LOOP; CLOSE cur_orders;

B. OPEN cur_orders; DECLARE cur_orders CURSOR FOR SELECT * FROM orders; FETCH cur_orders INTO rec; CLOSE cur_orders;

C. DECLARE cur_orders CURSOR FOR SELECT * FROM orders; FETCH cur_orders INTO rec; OPEN cur_orders; CLOSE cur_orders;

D. DECLARE cur_orders CURSOR FOR SELECT * FROM orders; OPEN cur_orders; FETCH ALL FROM cur_orders INTO rec; CLOSE cur_orders;

Solution

1. Step 1: Understand correct cursor usage in PL/pgSQL

   Declare cursor, open it, then loop fetching rows until no more rows, then close cursor.

2. Step 2: Analyze each option

   DECLARE cur_orders CURSOR FOR SELECT * FROM orders; OPEN cur_orders; LOOP FETCH cur_orders INTO rec; EXIT WHEN NOT FOUND; -- process rec END LOOP; CLOSE cur_orders; follows correct order and uses LOOP with EXIT WHEN NOT FOUND to process all rows. Others have wrong order or invalid FETCH ALL.

3. Final Answer:

   DECLARE cur_orders CURSOR FOR SELECT * FROM orders; OPEN cur_orders; LOOP FETCH cur_orders INTO rec; EXIT WHEN NOT FOUND; -- process rec END LOOP; CLOSE cur_orders; -> Option A

4. Quick Check:

   Declare, Open, Loop Fetch, Close = correct pattern [OK]

Hint: Use LOOP with FETCH and EXIT WHEN NOT FOUND [OK]

Common Mistakes:

• Opening cursor before declaring
• Fetching before opening
• Using FETCH ALL which is invalid for cursors