Why filtering behavior matters in PostgreSQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all rows where the age is greater than 30.

PostgreSQL

SELECT * FROM users WHERE age [1] 30;

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

A<

B=

C>

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' will select ages less than 30, which is wrong.

Using '=' will select only age exactly 30, not greater.

The '>' operator filters rows where age is greater than 30.

2fill in blank

medium

Complete the code to filter users whose name starts with 'A'.

PostgreSQL

SELECT * FROM users WHERE name [1] 'A%';

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

ALIKE

B=

CILIKE

DIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' will look for exact match 'A%', not pattern.

Using IN is for lists, not patterns.

The LIKE operator filters strings matching a pattern. 'A%' means names starting with 'A'.

3fill in blank

hard

Fix the error in the query to filter users with age not equal to 25.

PostgreSQL

SELECT * FROM users WHERE age [1] 25;

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

A<>

B!=

C==

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' causes syntax error.

Using '=' selects equal values, not 'not equal'.

In PostgreSQL, '<>' means 'not equal'. '!=' also works but '<>' is standard SQL.

4fill in blank

hard

Fill both blanks to select users with age between 20 and 30 inclusive.

PostgreSQL

SELECT * FROM users WHERE age [1] 20 AND age [2] 30;

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

A>=

B<

C<=

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' or '<' excludes boundary ages 20 or 30.

Mixing operators incorrectly causes wrong filtering.

Use '>=' for age at least 20 and '<=' for age at most 30 to include both ends.

5fill in blank

hard

Fill all three blanks to select users whose name starts with 'J', age is greater than 25, and city is not 'London'.

PostgreSQL

SELECT * FROM users WHERE name [1] 'J%' AND age [2] 25 AND city [3] 'London';

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

ALIKE

B>

C!=

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' instead of '!=' for city filters wrong rows.

Using '=' instead of LIKE for name pattern causes no matches.

Use LIKE for pattern matching name starting with 'J', '>' for age greater than 25, and '!=' for city not equal to 'London'.

Practice

1. What is the main purpose of the WHERE clause in a PostgreSQL query?

easy

A. To create a new table

B. To sort the results alphabetically

C. To join two tables together

D. To filter rows based on a condition

Solution

1. Step 1: Understand the role of WHERE clause

   The WHERE clause is used to specify conditions that rows must meet to be included in the result.

2. Step 2: Compare options with WHERE purpose

   Sorting is done by ORDER BY, joining by JOIN, and creating tables by CREATE TABLE, so these are incorrect.

3. Final Answer:

   To filter rows based on a condition -> Option D

4. Quick Check:

   WHERE filters rows = A [OK]

Hint: WHERE filters rows to get only what you want [OK]

Common Mistakes:

• Confusing WHERE with ORDER BY
• Thinking WHERE creates tables
• Mixing WHERE with JOIN

2. Which of the following is the correct syntax to filter rows where the column age is greater than 30?

easy

A. SELECT * FROM users FILTER age > 30;

B. SELECT * FROM users WHERE age > 30;

C. SELECT * FROM users HAVING age > 30;

D. SELECT * FROM users ORDER BY age > 30;

Solution

1. Step 1: Identify correct filtering syntax

   The WHERE clause is used to filter rows based on conditions like age > 30.

2. Step 2: Eliminate incorrect clauses

   FILTER is not valid here, HAVING is for groups, ORDER BY sorts results, so only WHERE is correct.

3. Final Answer:

   SELECT * FROM users WHERE age > 30; -> Option B

4. Quick Check:

   WHERE filters rows correctly = A [OK]

Hint: Use WHERE for row conditions, HAVING for groups [OK]

Common Mistakes:

• Using HAVING without GROUP BY
• Using FILTER instead of WHERE
• Confusing ORDER BY with filtering

3. Given the table employees with columns id, name, and salary, what will this query return?

SELECT name FROM employees WHERE salary >= 50000;

medium

A. All employee names with salary equal or greater than 50000

B. Syntax error due to missing GROUP BY

C. All employee names regardless of salary

D. All employee names with salary less than 50000

Solution

1. Step 1: Analyze the WHERE condition

   The condition salary >= 50000 filters employees with salary 50000 or more.

2. Step 2: Understand SELECT output

   Only the name column is selected for those filtered employees.

3. Final Answer:

   All employee names with salary equal or greater than 50000 -> Option A

4. Quick Check:

   WHERE salary >= 50000 filters names = D [OK]

Hint: WHERE filters rows; SELECT chooses columns [OK]

Common Mistakes:

• Thinking it returns all employees
• Confusing >= with <
• Expecting GROUP BY needed here

4. Identify the error in this query:

SELECT * FROM orders WHERE status = 'shipped' AND;

medium

A. Incorrect use of WHERE instead of HAVING

B. Missing ORDER BY clause

C. Missing condition after AND

D. Using * instead of column names

Solution

1. Step 1: Check WHERE clause syntax

   The WHERE clause ends with AND but no condition follows, causing syntax error.

2. Step 2: Validate other parts

   HAVING is not needed here, ORDER BY is optional, and * is valid to select all columns.

3. Final Answer:

   Missing condition after AND -> Option C

4. Quick Check:

   AND must be followed by condition = B [OK]

Hint: AND must always have a condition after it [OK]

Common Mistakes:

• Leaving AND without condition
• Confusing WHERE and HAVING
• Thinking ORDER BY is mandatory

5. You want to find customers who placed orders with total amount over 100 but only if their account is active. Which query correctly filters this?

SELECT customer_id FROM orders WHERE total > 100 AND account_status = 'active';

hard

A. This query correctly filters customers with total > 100 and active accounts

B. The query should use HAVING instead of WHERE for filtering

C. The query needs GROUP BY customer_id to filter correctly

D. Filtering on account_status is invalid in WHERE clause

Solution

1. Step 1: Understand filtering conditions

   Both conditions total > 100 and account_status = 'active' filter rows correctly in WHERE clause.

2. Step 2: Check if GROUP BY or HAVING needed

   Since no aggregation is used, GROUP BY or HAVING is not required here.

3. Final Answer:

   This query correctly filters customers with total > 100 and active accounts -> Option A

4. Quick Check:

   WHERE filters multiple conditions correctly = C [OK]

Hint: Use WHERE for multiple conditions without aggregation [OK]

Common Mistakes:

• Using HAVING without aggregation
• Adding unnecessary GROUP BY
• Thinking account_status can't be filtered in WHERE