Cursor declaration and usage in PostgreSQL - Mini Project: Build & Apply

Cursor Declaration and Usage in PostgreSQL

📖 Scenario: You are managing a small bookstore database. You want to process each book record one by one to check stock levels and prepare for restocking.

🎯 Goal: Build a PostgreSQL script that declares a cursor to select books with low stock, fetches each record one at a time, and processes them.

📋 What You'll Learn

Create a cursor named book_cursor selecting id, title, and stock from the books table where stock is less than 5

Declare an integer variable low_stock_threshold with value 5

Use a LOOP to fetch each row from book_cursor into variables book_id, book_title, and book_stock

Exit the loop when no more rows are found

Close the cursor after processing

💡 Why This Matters

🌍 Real World

Cursors help process large query results row by row, useful in inventory management or batch processing.

💼 Career

Database developers and administrators use cursors to handle complex data processing tasks efficiently.

1

Declare the cursor

Write a PostgreSQL DECLARE statement to create a cursor named book_cursor that selects id, title, and stock from the books table where stock < 5.

PostgreSQL

-- Declare the cursor named book_cursor
-- Your code here

Hint

Use DECLARE cursor_name CURSOR FOR SELECT ... syntax.

2

Declare the threshold variable

Declare an integer variable named low_stock_threshold and set it to 5.

PostgreSQL

DECLARE book_cursor CURSOR FOR SELECT id, title, stock FROM books WHERE stock < 5;
-- Declare low_stock_threshold variable
-- Your code here

Hint

Use variable_name TYPE := value; to declare and initialize.

3

Fetch rows using a loop

Write a LOOP that fetches each row from book_cursor into variables book_id, book_title, and book_stock. Exit the loop when no more rows are found.

PostgreSQL

DECLARE book_cursor CURSOR FOR SELECT id, title, stock FROM books WHERE stock < 5;
low_stock_threshold INTEGER := 5;

-- Start loop to fetch rows
-- Your code here

Hint

Use FETCH cursor_name INTO variables; and EXIT WHEN NOT FOUND; inside the loop.

4

Close the cursor

Add a statement to close the cursor named book_cursor after the loop.

PostgreSQL

DECLARE book_cursor CURSOR FOR SELECT id, title, stock FROM books WHERE stock < 5;
low_stock_threshold INTEGER := 5;

LOOP
    FETCH book_cursor INTO book_id, book_title, book_stock;
    EXIT WHEN NOT FOUND;
    -- Processing logic can go here
END LOOP;

-- Close the cursor
-- Your code here

Hint

Use CLOSE cursor_name; to release the cursor.

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