Performing operations on cursors in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a cursor named my_cursor for selecting all rows from the employees table.

PostgreSQL

DECLARE my_cursor CURSOR FOR SELECT * FROM [1];

Drag options to blanks, or click blank then click option'

Aemployees

Bdepartments

Csalaries

Dprojects

Attempts:

3 left

💡 Hint

Common Mistakes

Using a table name that does not exist.

Forgetting to specify the table after SELECT * FROM.

The cursor is declared to select all rows from the employees table.

2fill in blank

medium

Complete the code to open the cursor named my_cursor.

PostgreSQL

[1] my_cursor;

Drag options to blanks, or click blank then click option'

ACLOSE

BFETCH

COPEN

DDECLARE

Attempts:

3 left

💡 Hint

Common Mistakes

Using FETCH instead of OPEN to start the cursor.

Trying to DECLARE the cursor again instead of opening it.

The OPEN statement is used to start the cursor so it can be used to fetch rows.

3fill in blank

hard

Fix the error in the code to fetch one row from the cursor my_cursor into the variable emp_record.

PostgreSQL

FETCH [1] my_cursor INTO emp_record;

Drag options to blanks, or click blank then click option'

ANEXT

BALL

CPRIOR

DFIRST

Attempts:

3 left

💡 Hint

Common Mistakes

Using ALL fetches all rows, which is not valid syntax here.

Using PRIOR or FIRST fetches rows in different directions, which may cause errors.

The NEXT option fetches the next row from the cursor, which is the default and correct choice here.

4fill in blank

hard

Fill both blanks to close the cursor and then deallocate it.

PostgreSQL

[1] my_cursor;
[2] my_cursor;

Drag options to blanks, or click blank then click option'

ACLOSE

BOPEN

CDEALLOCATE

DFETCH

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to deallocate before closing the cursor.

Using OPEN or FETCH instead of CLOSE or DEALLOCATE.

First, the cursor is CLOSEd to stop it, then DEALLOCATEd to free resources.

5fill in blank

hard

Fill all three blanks to declare a cursor named emp_cursor for employees with salary over 50000, open it, and fetch one row into emp_rec.

PostgreSQL

DECLARE [1] CURSOR FOR SELECT * FROM employees WHERE salary [2] 50000;
[3] [1];
FETCH NEXT [1] INTO emp_rec;

Drag options to blanks, or click blank then click option'

Aemp_cursor

B>

COPEN

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong cursor name or inconsistent names.

Using < or other operators instead of > for salary condition.

Forgetting to open the cursor before fetching.

The cursor is named emp_cursor. It selects employees with salary greater than 50000. Then it is OPENed before fetching.

Practice

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

easy

A. To process query results one row at a time

B. To speed up query execution by parallel processing

C. To permanently store query results in a table

D. To automatically optimize query plans

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

   Only To process query results one row at a time correctly describes this row-wise processing purpose.

3. Final Answer:

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

4. Quick Check:

   Cursor = row-by-row processing [OK]

Hint: Cursors process rows one by one, not all at once [OK]

Common Mistakes:

• Thinking cursors speed up queries automatically
• Confusing cursors with temporary tables
• Assuming cursors optimize query plans

2. Which of the following is the correct syntax to declare a cursor named cur1 for a SELECT query in PostgreSQL?

easy

A. OPEN cur1 CURSOR FOR SELECT * FROM employees;

B. DECLARE cur1 CURSOR FOR SELECT * FROM employees;

C. FETCH cur1 CURSOR FOR SELECT * FROM employees;

D. CREATE CURSOR cur1 FOR SELECT * FROM employees;

Solution

1. Step 1: Recall cursor declaration syntax

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

2. Step 2: Match syntax with options

   DECLARE cur1 CURSOR FOR SELECT * FROM employees; matches the correct DECLARE syntax; others misuse OPEN, FETCH, or CREATE.

3. Final Answer:

   DECLARE cur1 CURSOR FOR SELECT * FROM employees; -> Option B

4. Quick Check:

   DECLARE cursor FOR query [OK]

Hint: Use DECLARE to define cursor before OPEN or FETCH [OK]

Common Mistakes:

• Using OPEN instead of DECLARE to define cursor
• Trying to FETCH during declaration
• Using CREATE CURSOR which is invalid

3. Consider this PostgreSQL code snippet:

DECLARE cur CURSOR FOR SELECT id FROM users ORDER BY id;
OPEN cur;
FETCH NEXT FROM cur;
FETCH NEXT FROM cur;
CLOSE cur;

What will be the output of the two FETCH commands if the users table has ids 10, 20, 30 in ascending order?

medium

A. First FETCH returns 10, second FETCH returns 20

B. First FETCH returns 20, second FETCH returns 30

C. Both FETCH commands return 10

D. First FETCH returns 30, second FETCH returns NULL

Solution

1. Step 1: Understand cursor order and FETCH

   The cursor selects ids ordered by id: 10, 20, 30. FETCH NEXT returns rows sequentially.

2. Step 2: Trace FETCH commands

   First FETCH returns the first row: 10. Second FETCH returns the next row: 20.

3. Final Answer:

   First FETCH returns 10, second FETCH returns 20 -> Option A

4. Quick Check:

   FETCH NEXT returns rows in order [OK]

Hint: FETCH NEXT returns rows in declared order one by one [OK]

Common Mistakes:

• Assuming FETCH skips rows
• Confusing FETCH NEXT with FETCH ALL
• Expecting FETCH to return NULL before end

4. Given this code snippet:

DECLARE cur CURSOR FOR SELECT name FROM products;
OPEN cur;
FETCH cur;
CLOSE cur;
FETCH cur;

What error will occur when running this code?

medium

A. ERROR: cursor "cur" does not exist

B. No error, FETCH returns next row

C. ERROR: syntax error near FETCH

D. ERROR: cursor "cur" is not open

Solution

1. Step 1: Analyze cursor lifecycle

   Cursor is declared and opened, then FETCH is called once, then cursor is closed.

2. Step 2: Identify error on second FETCH

   After CLOSE, cursor is not open, so FETCH causes "cursor is not open" error.

3. Final Answer:

   ERROR: cursor "cur" is not open -> Option D

4. Quick Check:

   FETCH after CLOSE causes 'not open' error [OK]

Hint: Cannot FETCH after CLOSE; cursor must be open [OK]

Common Mistakes:

• Trying to FETCH after cursor is closed
• Expecting FETCH to reopen cursor automatically
• Confusing 'not open' with 'does not exist' error

5. You want to process a large table orders row by row using a cursor in a PL/pgSQL function. Which sequence of commands correctly opens the cursor, fetches all rows one by one, and closes it after processing?

hard

A. DECLARE cur CURSOR FOR SELECT * FROM orders; OPEN cur; FETCH ALL FROM cur; CLOSE cur;

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

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

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

Solution

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

   Declare cursor, open it, then loop fetching rows until no more rows (EXIT WHEN NOT FOUND), then close cursor.

2. Step 2: Evaluate options

   DECLARE cur CURSOR FOR SELECT * FROM orders; OPEN cur; LOOP FETCH cur INTO rec; EXIT WHEN NOT FOUND; -- process rec END LOOP; CLOSE cur; correctly shows DECLARE, OPEN, LOOP with FETCH and EXIT, then CLOSE. Others misuse order or FETCH ALL which fetches all rows at once.

3. Final Answer:

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

4. Quick Check:

   Cursor loop with FETCH and EXIT WHEN NOT FOUND [OK]

Hint: Use LOOP with FETCH and EXIT WHEN NOT FOUND to process cursor rows [OK]

Common Mistakes:

• Opening cursor after FETCH
• Using FETCH ALL instead of looping FETCH
• Closing cursor before processing all rows