SELECT with PostgreSQL-specific features - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all columns from the table named 'employees'.

PostgreSQL

SELECT [1] FROM employees;

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

A*

BALL

Ccolumns

Deverything

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'ALL' instead of '*'

Writing 'columns' or 'everything' which are not valid SQL keywords

Using * selects all columns from the table.

2fill in blank

medium

Complete the code to select the first 5 rows from the 'products' table using PostgreSQL syntax.

PostgreSQL

SELECT * FROM products [1] 5;

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ATOP

BLIMIT

CFETCH

DROWS

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'TOP' which is not valid in PostgreSQL

Using 'FETCH' or 'ROWS' without the full syntax

PostgreSQL uses LIMIT to restrict the number of rows returned.

3fill in blank

hard

Fix the error in the query to select distinct city names from the 'customers' table.

PostgreSQL

SELECT DISTINCT [1] FROM customers;

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Acity_name

Bcities

Ccity

Dcustomer_city

Attempts:

3 left

💡 Hint

Common Mistakes

Using plural or incorrect column names

Using column names that do not exist in the table

The column name is city. Using the correct column name avoids errors.

4fill in blank

hard

Fill both blanks to select the name and the length of the name from the 'users' table using PostgreSQL functions.

PostgreSQL

SELECT name, [1](name) AS name_length FROM users WHERE [2](name) > 5;

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ALENGTH

BLEN

CSIZE

DCOUNT

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'LEN' which is not valid in PostgreSQL

Using 'SIZE' or 'COUNT' which do not measure string length

PostgreSQL uses the LENGTH function to get the number of characters in a string.

5fill in blank

hard

Fill all three blanks to select the name in uppercase, the length of the name, and filter names longer than 3 characters from the 'members' table.

PostgreSQL

SELECT [1](name) AS upper_name, [2](name) AS name_len FROM members WHERE [3](name) > 3;

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

AUPPER

BLENGTH

DLOWER

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'LOWER' instead of 'UPPER' for uppercase conversion

Using 'LEN' instead of 'LENGTH' for string length

UPPER converts text to uppercase, and LENGTH returns the number of characters in the string.

Practice

1. What does the LIMIT clause do in a PostgreSQL SELECT query?

easy

A. It combines rows from two tables.

B. It restricts the number of rows returned by the query.

C. It removes duplicate rows from the result.

D. It sorts the rows in ascending order.

Solution

1. Step 1: Understand the purpose of LIMIT

   The LIMIT clause is used to specify the maximum number of rows to return from a query result.

2. Step 2: Compare with other options

   Sorting is done by ORDER BY, removing duplicates by DISTINCT, and combining tables by JOIN, so these are not correct.

3. Final Answer:

   It restricts the number of rows returned by the query. -> Option B

4. Quick Check:

   LIMIT = restrict rows [OK]

Hint: LIMIT caps rows returned, like a page size [OK]

Common Mistakes:

• Confusing LIMIT with ORDER BY
• Thinking LIMIT removes duplicates
• Mixing LIMIT with JOIN functionality

2. Which of the following is the correct syntax to select distinct city names from a table called customers?

easy

A. SELECT city FROM DISTINCT customers;

B. SELECT city DISTINCT FROM customers;

C. SELECT DISTINCT city FROM customers;

D. SELECT city FROM customers DISTINCT;

Solution

1. Step 1: Recall correct DISTINCT syntax

   In PostgreSQL, DISTINCT comes immediately after SELECT and before the column list.

2. Step 2: Validate each option

   SELECT DISTINCT city FROM customers; correctly places DISTINCT after SELECT. Options A, B, and D misuse DISTINCT placement causing syntax errors.

3. Final Answer:

   SELECT DISTINCT city FROM customers; -> Option C

4. Quick Check:

   SELECT DISTINCT = correct syntax [OK]

Hint: DISTINCT always follows SELECT keyword [OK]

Common Mistakes:

• Placing DISTINCT after column names
• Using DISTINCT after FROM clause
• Misordering keywords causing syntax errors

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

SELECT name FROM products ORDER BY price DESC LIMIT 2;

medium

A. The two products with the lowest prices.

B. An error because LIMIT cannot be used with ORDER BY.

C. All products sorted by price descending.

D. The two products with the highest prices.

Solution

1. Step 1: Understand ORDER BY with DESC

   ORDER BY price DESC sorts products from highest to lowest price.

2. Step 2: Apply LIMIT 2

   LIMIT 2 restricts the output to only the first two rows after sorting, so the two most expensive products.

3. Final Answer:

   The two products with the highest prices. -> Option D

4. Quick Check:

   ORDER BY DESC + LIMIT = top rows [OK]

Hint: ORDER BY DESC + LIMIT picks top items [OK]

Common Mistakes:

• Thinking LIMIT returns lowest prices
• Believing LIMIT causes syntax error with ORDER BY
• Ignoring DESC means descending order

4. Identify the error in this PostgreSQL query:

SELECT DISTINCT name, price FROM products LIMIT;

medium

A. LIMIT requires a number to specify how many rows to return.

B. DISTINCT cannot be used with multiple columns.

C. SELECT must include WHERE clause when using LIMIT.

D. The table name is missing.

Solution

1. Step 1: Check LIMIT usage

   LIMIT must be followed by a number indicating how many rows to return; here it is missing.

2. Step 2: Validate other parts

   DISTINCT can be used with multiple columns, WHERE is optional, and table name is present as products.

3. Final Answer:

   LIMIT requires a number to specify how many rows to return. -> Option A

4. Quick Check:

   LIMIT needs a number [OK]

Hint: Always put a number after LIMIT [OK]

Common Mistakes:

• Omitting number after LIMIT
• Thinking DISTINCT can't use multiple columns
• Assuming WHERE is mandatory with LIMIT

5. You want to get the third and fourth highest salaries from the employees table with column salary. Which query correctly achieves this in PostgreSQL?

hard

A. SELECT salary FROM employees ORDER BY salary DESC LIMIT 2 OFFSET 2;

B. SELECT salary FROM employees ORDER BY salary ASC LIMIT 2 OFFSET 3;

C. SELECT DISTINCT salary FROM employees LIMIT 2 OFFSET 3 ORDER BY salary DESC;

D. SELECT salary FROM employees LIMIT 2 OFFSET 2 ORDER BY salary DESC;

Solution

1. Step 1: Understand OFFSET and LIMIT

   OFFSET 2 skips the first two highest salaries, LIMIT 2 returns the next two rows.

2. Step 2: Check ORDER BY direction

   ORDER BY salary DESC sorts salaries from highest to lowest, so OFFSET 2 skips top two highest.

3. Step 3: Validate options

   SELECT salary FROM employees ORDER BY salary DESC LIMIT 2 OFFSET 2; correctly orders DESC, uses LIMIT 2 and OFFSET 2. SELECT salary FROM employees ORDER BY salary ASC LIMIT 2 OFFSET 3; orders ASC (wrong direction), C misplaces ORDER BY, D places ORDER BY after LIMIT (syntax error).

4. Final Answer:

   SELECT salary FROM employees ORDER BY salary DESC LIMIT 2 OFFSET 2; -> Option A

5. Quick Check:

   OFFSET skips rows, LIMIT picks next [OK]

Hint: Use OFFSET to skip, LIMIT to pick rows [OK]

Common Mistakes:

• Using ASC instead of DESC for highest values
• Placing ORDER BY after LIMIT
• Confusing OFFSET number with row count