SELECT with expressions and calculations in SQL - Step-by-Step Execution

Concept Flow - SELECT with expressions and calculations

Start: SELECT statement

↓

Identify columns and expressions

↓

Calculate expressions for each row

↓

Build result set with calculated values

↓

Return result set to user

The SELECT statement processes each row, calculates expressions, and returns the results.

Execution Sample

SQL

SELECT price, quantity, price * quantity AS total
FROM sales;

This query selects price and quantity columns and calculates total as price times quantity for each row.

Execution Table

Step	Row Data (price, quantity)	Expression Calculated	Output Row
1	(10, 2)	10 * 2 = 20	(10, 2, 20)
2	(5, 5)	5 * 5 = 25	(5, 5, 25)
3	(8, 3)	8 * 3 = 24	(8, 3, 24)
4	No more rows	No calculation	End of result set

💡 All rows processed, query returns result set with calculated totals.

Variable Tracker

Variable	Start	After Row 1	After Row 2	After Row 3	Final
price	N/A	10	5	8	N/A
quantity	N/A	2	5	3	N/A
total	N/A	20	25	24	N/A

Key Moments - 2 Insights

Why does the expression 'price * quantity' get calculated for each row separately?

Because the SELECT processes each row individually, calculating expressions using that row's column values, as shown in execution_table rows 1 to 3.

What happens if there are no rows in the table?

The query returns an empty result set immediately, as shown in execution_table row 4 where no more rows exist.

Visual Quiz - 3 Questions

Look at the execution table, what is the 'total' value at step 2?

A 10

B 20

C 25

D 5

💡 Hint

Check the 'Expression Calculated' column at step 2 in the execution_table.

At which step does the query finish processing all rows?

A Step 3

B Step 4

C Step 2

D Step 1

💡 Hint

Look for the row indicating 'No more rows' in the execution_table.

If the quantity in the second row changed from 5 to 4, what would be the new total at step 2?

A 20

B 25

C 24

D 21

💡 Hint

Multiply price and quantity values from variable_tracker for row 2.

Concept Snapshot

SELECT with expressions lets you calculate new values from columns.
Syntax: SELECT col1, col2, expression AS alias FROM table;
Each row's expression is calculated separately.
Results include original columns plus calculated values.
Useful for totals, differences, or any math on data.

Full Transcript

This visual execution shows how a SELECT query with expressions works. The query selects price and quantity columns from a sales table and calculates a new column total by multiplying price and quantity for each row. The execution table traces each row: it shows the input values, the calculation done, and the output row with the new total. Variables price, quantity, and total change as each row is processed. The query stops when no more rows remain. Key points include that expressions are calculated per row and that an empty table returns no results. The quiz tests understanding of these steps and calculations.

Practice

1. What does the SQL expression SELECT price * quantity AS total_cost FROM sales; do?

easy

A. Calculates total cost by multiplying price and quantity for each row.

B. Adds price and quantity values together for each row.

C. Divides price by quantity for each row.

D. Filters rows where price equals quantity.

Solution

1. Step 1: Understand the multiplication in SELECT

   The expression multiplies the values in columns price and quantity for each row.

2. Step 2: Recognize the alias usage

   The result of multiplication is renamed as total_cost for clarity in output.

3. Final Answer:

   Calculates total cost by multiplying price and quantity for each row. -> Option A

4. Quick Check:

   price * quantity = total_cost [OK]

Hint: Look for * operator to identify multiplication [OK]

Common Mistakes:

• Confusing multiplication with addition or division
• Ignoring the AS alias
• Thinking it filters rows

2. Which of the following is the correct syntax to calculate the average price and rename it as avg_price?

easy

A. SELECT AVG(price) AS avg_price FROM products;

B. SELECT AVG(price) avg_price FROM products;

C. SELECT AVG(price) TO avg_price FROM products;

D. SELECT AVG(price) RENAME avg_price FROM products;

Solution

1. Step 1: Identify correct alias syntax

   In SQL, the keyword AS is used to rename columns or expressions in SELECT.

2. Step 2: Check each option's syntax

   SELECT AVG(price) AS avg_price FROM products; uses AVG(price) AS avg_price which is the correct syntax; others use invalid keywords or omit AS.

3. Final Answer:

   SELECT AVG(price) AS avg_price FROM products; -> Option A

4. Quick Check:

   Use AS to rename columns [OK]

Hint: Use AS keyword to rename calculated columns [OK]

Common Mistakes:

• Omitting AS keyword
• Using TO or RENAME instead of AS
• Misplacing parentheses

3. Given a table orders with columns price and quantity, what is the output of this query?

SELECT price, quantity, price * quantity AS total FROM orders WHERE price = 10;

medium

A. All rows with total calculated, ignoring price filter.

B. Syntax error due to missing GROUP BY.

C. Rows where price is 10, showing price, quantity, and total (price*quantity).

D. Rows where quantity equals 10.

Solution

1. Step 1: Understand the WHERE clause effect

   The query filters rows to only those where price equals 10.

2. Step 2: Calculate total for filtered rows

   For each filtered row, it shows price, quantity, and total which is price multiplied by quantity.

3. Final Answer:

   Rows where price is 10, showing price, quantity, and total (price*quantity). -> Option C

4. Quick Check:

   WHERE price=10 filters rows; total = price*quantity [OK]

Hint: Check WHERE filters before calculations [OK]

Common Mistakes:

• Ignoring WHERE clause filtering
• Expecting all rows instead of filtered
• Thinking GROUP BY is needed here

4. Identify the error in this SQL query:

SELECT price + quantity AS total FROM sales WHERE total > 100;

medium

A. Missing FROM clause in the query.

B. You cannot use alias 'total' in WHERE clause directly.

C. The + operator is invalid for numeric addition.

D. AS keyword cannot be used for aliasing.

Solution

1. Step 1: Understand alias scope in SQL

   Aliases defined in SELECT cannot be used in WHERE clause because WHERE is evaluated before SELECT.

2. Step 2: Identify correct fix

   Use the full expression (price + quantity) in WHERE or use a subquery/CTE to filter by total.

3. Final Answer:

   You cannot use alias 'total' in WHERE clause directly. -> Option B

4. Quick Check:

   Alias not usable in WHERE [OK]

Hint: Aliases appear after WHERE; can't use them there [OK]

Common Mistakes:

• Using alias in WHERE clause
• Thinking + is invalid operator
• Ignoring query evaluation order

5. You want to calculate the discounted price for products with a 10% discount and show it as discounted_price. Which query correctly applies the discount and rounds the result to 2 decimals?

hard

A. SELECT price - (price * 0.10) AS discounted_price FROM products;

B. SELECT price * 0.90 AS discounted_price FROM products;

C. SELECT ROUND(price * 0.90) AS discounted_price FROM products;

D. SELECT ROUND(price - (price * 0.10), 2) AS discounted_price FROM products;

Solution

1. Step 1: Calculate 10% discount correctly

   Discounted price is price minus 10% of price, which is price - (price * 0.10) or price * 0.90.

2. Step 2: Round the result to 2 decimals

   Use ROUND(expression, 2) to round to two decimal places for currency formatting.

3. Final Answer:

   SELECT ROUND(price - (price * 0.10), 2) AS discounted_price FROM products; -> Option D

4. Quick Check:

   Use ROUND(..., 2) for 2 decimals [OK]

Hint: Use ROUND(expression, 2) to round decimals [OK]

Common Mistakes:

• Not rounding the result
• Rounding without specifying decimals
• Using incorrect discount calculation