WHERE with OR operator in SQL - Step-by-Step Execution

Concept Flow - WHERE with OR operator

Start Query

↓

Scan Table Rows

↓

Evaluate WHERE Condition

↓

Condition 1 True?

Yes→Include Row

↓

Condition 2 True?

Yes→Include Row

↓

Exclude Row

↓

Return Filtered Rows

↓

End Query

The query scans each row, checks if either condition in the OR is true, includes the row if yes, otherwise excludes it.

Execution Sample

SQL

SELECT * FROM employees
WHERE department = 'Sales' OR salary > 70000;

This query selects employees who are either in the Sales department or have a salary greater than 70000.

Execution Table

Step	Row Data	Condition: department='Sales'	Condition: salary>70000	OR Result	Include Row?
1	{id:1, department:'Sales', salary:65000}	True	False	True	Yes
2	{id:2, department:'HR', salary:72000}	False	True	True	Yes
3	{id:3, department:'IT', salary:60000}	False	False	False	No
4	{id:4, department:'Sales', salary:75000}	True	True	True	Yes
5	{id:5, department:'Marketing', salary:50000}	False	False	False	No
Exit	All rows processed	-	-	-	Query ends

💡 All rows checked; rows included if either condition is true.

Variable Tracker

Variable	Start	After Row 1	After Row 2	After Row 3	After Row 4	After Row 5	Final
department='Sales'	N/A	True	False	False	True	False	N/A
salary>70000	N/A	False	True	False	True	False	N/A
OR Result	N/A	True	True	False	True	False	N/A
Include Row?	N/A	Yes	Yes	No	Yes	No	N/A

Key Moments - 3 Insights

Why does the query include a row if only one of the conditions is true?

Because the OR operator returns true if at least one condition is true, as shown in the execution_table rows 1 and 2 where only one condition is true but the row is included.

What happens if both conditions are false for a row?

The OR result is false, so the row is excluded. This is shown in rows 3 and 5 of the execution_table.

Does the OR operator check both conditions even if the first is true?

In SQL, the OR operator evaluates conditions until it finds one true condition. However, for clarity, the table shows both conditions evaluated for each row.

Visual Quiz - 3 Questions

Look at the execution_table, what is the OR Result for row 3?

A True

B False

C Unknown

D Depends on database

💡 Hint

Check the 'OR Result' column for Step 3 in the execution_table.

At which step does the condition 'salary > 70000' become true for the first time?

A Step 2

B Step 1

C Step 4

D Step 5

💡 Hint

Look at the 'Condition: salary>70000' column in the execution_table.

If the OR operator was replaced with AND, which row(s) would be included?

A Rows 1 and 4

B Rows 2 and 4

C Only Row 4

D All rows

💡 Hint

Check which rows have both conditions true in the execution_table.

Concept Snapshot

WHERE with OR operator:
- Syntax: WHERE condition1 OR condition2
- Returns rows where at least one condition is true
- Evaluates each row's conditions
- Includes row if OR result is true
- Useful for filtering multiple criteria

Full Transcript

This visual execution shows how the SQL WHERE clause with the OR operator works. The query scans each row in the employees table and checks two conditions: if the department is 'Sales' or if the salary is greater than 70000. For each row, it evaluates both conditions and then applies the OR operator. If either condition is true, the row is included in the result. The execution table lists each row's data, condition results, the OR result, and whether the row is included. Key moments clarify why rows with one true condition are included and what happens when both are false. The quiz tests understanding of condition evaluation and the effect of changing OR to AND.

Practice

1. What does the SQL WHERE clause with the OR operator do?

easy

A. It deletes rows that meet any condition.

B. It selects rows that meet at least one of the conditions.

C. It selects rows that meet all conditions simultaneously.

D. It updates rows that meet none of the conditions.

Solution

1. Step 1: Understand the WHERE clause

   The WHERE clause filters rows based on conditions.

2. Step 2: Understand the OR operator

   OR means if any condition is true, the row is selected.

3. Final Answer:

   It selects rows that meet at least one of the conditions. -> Option B

4. Quick Check:

   WHERE with OR = any condition true [OK]

Hint: OR means any condition true selects the row [OK]

Common Mistakes:

• Thinking OR requires all conditions true
• Confusing OR with AND
• Assuming OR deletes or updates rows

2. Which of the following SQL queries correctly uses the WHERE clause with the OR operator to find employees in department 10 or 20?

easy

A. SELECT * FROM employees WHERE department_id = 10 AND 20;

B. SELECT * FROM employees WHERE department_id = (10 OR 20);

C. SELECT * FROM employees WHERE department_id = 10 OR department_id = 20;

D. SELECT * FROM employees WHERE department_id IN 10 OR 20;

Solution

1. Step 1: Check correct OR syntax

   OR must connect full conditions: column = value OR column = value.

2. Step 2: Analyze each option

   A uses AND incorrectly; C uses invalid syntax; D misuses IN without parentheses.

3. Final Answer:

   SELECT * FROM employees WHERE department_id = 10 OR department_id = 20; -> Option C

4. Quick Check:

   Correct OR syntax = SELECT * FROM employees WHERE department_id = 10 OR department_id = 20; [OK]

Hint: Use OR between full conditions, not partial expressions [OK]

Common Mistakes:

• Using AND instead of OR
• Writing OR inside parentheses incorrectly
• Misusing IN without parentheses

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

SELECT * FROM products WHERE category = 'Books' OR category = 'Toys';

medium

A. Products in either 'Books' or 'Toys' categories.

B. Only products in both 'Books' and 'Toys' categories.

C. All products except those in 'Books' or 'Toys'.

D. No products because OR is invalid here.

Solution

1. Step 1: Understand the OR condition

   The query selects rows where category is 'Books' or 'Toys'.

2. Step 2: Determine matching rows

   Rows with category 'Books' or 'Toys' will be included, others excluded.

3. Final Answer:

   Products in either 'Books' or 'Toys' categories. -> Option A

4. Quick Check:

   OR selects rows matching any condition [OK]

Hint: OR includes rows matching any listed condition [OK]

Common Mistakes:

• Thinking OR means both conditions must be true
• Assuming OR excludes matching rows
• Believing OR causes syntax error

4. Identify the error in this SQL query:

SELECT * FROM orders WHERE status = 'Pending' OR;

medium

A. Missing condition after OR operator.

B. Incorrect use of WHERE clause.

C. OR operator should be AND.

D. No error, query is valid.

Solution

1. Step 1: Analyze the WHERE clause

   The WHERE clause ends with OR but no condition follows it.

2. Step 2: Understand OR syntax

   OR must connect two conditions; missing second condition causes syntax error.

3. Final Answer:

   Missing condition after OR operator. -> Option A

4. Quick Check:

   OR needs two conditions [OK]

Hint: OR must connect two conditions; don't leave it hanging [OK]

Common Mistakes:

• Leaving OR at end without condition
• Confusing OR with AND syntax
• Ignoring syntax errors in WHERE clause

5. You have a table students with columns id, grade, and city. Write a query to select students who have grade 'A' or live in 'New York' or 'Los Angeles'. Which query is correct?

hard

A. SELECT * FROM students WHERE grade = 'A' OR city = 'New York' OR 'Los Angeles';

B. SELECT * FROM students WHERE grade = 'A' AND city = 'New York' OR 'Los Angeles';

C. SELECT * FROM students WHERE grade = 'A' OR city = 'New York' AND city = 'Los Angeles';

D. SELECT * FROM students WHERE grade = 'A' OR city IN ('New York', 'Los Angeles');

Solution

1. Step 1: Understand the conditions

   Select students with grade 'A' OR city 'New York' OR city 'Los Angeles'.

2. Step 2: Analyze each query

   A has syntax error (missing column in last condition); B has syntax error and wrong logic; C has incorrect logic due to AND; D is correct and concise using IN.

3. Final Answer:

   SELECT * FROM students WHERE grade = 'A' OR city IN ('New York', 'Los Angeles'); -> Option D

4. Quick Check:

   Use OR with IN for multiple values [OK]

Hint: Use OR with IN for multiple city values [OK]

Common Mistakes:

• Using AND between city conditions
• Omitting column name after OR
• Syntax errors with OR and IN