Cursor declaration and usage in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

PostgreSQL

DECLARE mycursor 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

Choosing a table unrelated to employees.

Misspelling the table name.

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

2fill in blank

medium

Complete the code to open the cursor named mycursor.

PostgreSQL

OPEN [1];

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

Acur_emp

Bcursor1

Cmycursor

Demp_cursor

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different cursor name than declared.

Forgetting to open the cursor before fetching.

The cursor named mycursor is opened to start fetching rows.

3fill in blank

hard

Fix the error in the FETCH statement to retrieve the next row from the cursor.

PostgreSQL

FETCH [1] FROM mycursor;

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

Anext

Bfirst

Cprevious

Dlast

Attempts:

3 left

💡 Hint

Common Mistakes

Using FETCH FIRST or FETCH LAST which do not fetch the next row.

Using FETCH PREVIOUS which moves backward.

The FETCH NEXT statement retrieves the next row from the cursor.

4fill in blank

hard

Fill both blanks to declare and open a cursor named emp_cursor selecting employee_id and name from employees.

PostgreSQL

DECLARE [1] CURSOR FOR SELECT employee_id, name FROM employees;
OPEN [2];

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

Aemp_cursor

Bmycursor

Ccursor_emp

Demployee_cur

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for declaration and opening.

Misspelling the cursor name.

The cursor is declared and opened using the same name emp_cursor.

5fill in blank

hard

Fill all three blanks to fetch the first row from cursor emp_cursor into variables emp_id and emp_name.

PostgreSQL

FETCH [1] FROM emp_cursor INTO [2], [3];

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

Anext

Bemp_id

Cemp_name

Dfirst

Attempts:

3 left

💡 Hint

Common Mistakes

Using FETCH NEXT instead of FETCH FIRST.

Swapping variable names or using undeclared variables.

The FETCH FIRST statement retrieves the first row, storing values into variables emp_id and emp_name.

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