WHERE clause filtering in MySQL - Step-by-Step Execution

Concept Flow - WHERE clause filtering

Start with full table

↓

Apply WHERE condition

↓

Check each row

↓

Keep

↓

Return filtered rows

The WHERE clause filters rows by checking each row against a condition, keeping only those that match.

Execution Sample

MySQL

SELECT * FROM employees
WHERE department = 'Sales';

This query selects all rows from the employees table where the department is 'Sales'.

Execution Table

Step	Row ID	department value	Condition (department = 'Sales')	Action
1	1	Sales	True	Keep row
2	2	HR	False	Discard row
3	3	Sales	True	Keep row
4	4	Marketing	False	Discard row
5	5	Sales	True	Keep row
6	End			No more rows to check

💡 All rows checked; only rows with department = 'Sales' are kept.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
Filtered Rows Count	0	1	1	2	2	3	3

Key Moments - 2 Insights

Why are some rows discarded even though they exist in the table?

Rows are discarded because their department value does not match 'Sales', as shown in execution_table rows 2 and 4 where the condition is False.

Does the WHERE clause change the original table data?

No, the WHERE clause only filters rows for the query result; it does not modify the original table, as seen by checking all rows but only keeping some.

Visual Quiz - 3 Questions

Look at the execution table, what happens at step 3?

A Row 3 is discarded

B Row 3 is updated

C Row 3 is kept

D Row 3 is skipped

💡 Hint

Check the 'Action' column at step 3 in the execution_table.

At which step does the condition become False for the first time?

A Step 1

B Step 2

C Step 4

D Step 5

💡 Hint

Look at the 'Condition' column in execution_table rows 1-5.

If the WHERE condition was changed to department = 'HR', how would the filtered rows count change after step 5?

A It would be 1

B It would be 3

C It would be 0

D It would be 5

💡 Hint

Refer to variable_tracker and consider which rows have department 'HR'.

Concept Snapshot

WHERE clause filters rows in a table.
Syntax: SELECT * FROM table WHERE condition;
Checks each row; keeps rows where condition is true.
Does not change original data.
Useful to get specific data from large tables.

Full Transcript

The WHERE clause filtering starts with the full table and applies a condition to each row. Each row is checked if it meets the condition. Rows that meet the condition are kept, others are discarded. For example, in the query selecting employees where department equals 'Sales', only rows with 'Sales' in the department column are kept. The execution table shows each step checking rows and deciding to keep or discard. The variable tracker counts how many rows are kept after each step. Beginners often wonder why some rows disappear; it's because they don't meet the condition. Also, the WHERE clause does not change the table data, it only filters the output. Visual quizzes help reinforce understanding by asking about specific steps and outcomes.

Practice

1. What does the WHERE clause do in a SQL query?

easy

A. Filters rows based on a specified condition

B. Sorts the rows in ascending order

C. Joins two tables together

D. Creates a new table in the database

Solution

1. Step 1: Understand the purpose of WHERE clause

   The WHERE clause is used to filter rows that meet a specific condition in a SELECT query.

2. Step 2: Differentiate from other SQL clauses

   Sorting is done by ORDER BY, joining by JOIN, and creating tables by CREATE TABLE, so these are not correct.

3. Final Answer:

   Filters rows based on a specified condition -> Option A

4. Quick Check:

   WHERE clause = filters rows [OK]

Hint: WHERE filters rows by condition, not sorting or joining [OK]

Common Mistakes:

• Confusing WHERE with ORDER BY
• Thinking WHERE creates or joins tables
• Assuming WHERE changes table structure

2. Which of the following is the correct syntax to select all columns from a table named employees where the age is greater than 30?

easy

A. SELECT * FROM employees WHERE age => 30;

B. SELECT * FROM employees WHERE age > 30;

C. SELECT * employees WHERE age > 30;

D. SELECT * FROM employees WHERE age < 30;

Solution

1. Step 1: Check the basic SELECT syntax

   The correct syntax to select all columns is SELECT * FROM table_name.

2. Step 2: Verify the WHERE condition syntax

   The comparison operator for greater than is >, not =>. Also, the condition should match the question (age > 30).

3. Final Answer:

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

4. Quick Check:

   Correct syntax with > operator [OK]

Hint: Use > for greater than, syntax: SELECT * FROM table WHERE condition [OK]

Common Mistakes:

• Using => instead of >
• Omitting FROM keyword
• Using wrong comparison operator

3. Given the table products with columns id, name, and price, what will the query SELECT name FROM products WHERE price <= 50; return?

medium

A. All product names with price less than or equal to 50

B. All product names with price greater than 50

C. All product names regardless of price

D. Syntax error due to <= operator

Solution

1. Step 1: Understand the WHERE condition

   The condition price <= 50 means selecting rows where price is less than or equal to 50.

2. Step 2: Identify the selected columns

   The query selects only the name column from those filtered rows.

3. Final Answer:

   All product names with price less than or equal to 50 -> Option A

4. Quick Check:

   price <= 50 filters products [OK]

Hint: <= means less than or equal, filters rows accordingly [OK]

Common Mistakes:

• Confusing <= with >= operator
• Assuming it returns all products
• Thinking <= causes syntax error

4. Identify the error in the following query: SELECT * FROM orders WHERE status = 'shipped' OR;

medium

A. Incorrect use of single quotes around 'shipped'

B. Using * instead of column names

C. Missing semicolon at the end

D. Missing condition after OR operator

Solution

1. Step 1: Analyze the WHERE clause

   The WHERE clause ends with OR but has no condition after it, which is invalid syntax.

2. Step 2: Check other parts of the query

   Single quotes around 'shipped' are correct, semicolon is optional, and * is valid to select all columns.

3. Final Answer:

   Missing condition after OR operator -> Option D

4. Quick Check:

   OR must be followed by a condition [OK]

Hint: OR must always have conditions on both sides [OK]

Common Mistakes:

• Leaving OR without a following condition
• Thinking quotes around strings cause error
• Assuming semicolon is mandatory

5. You have a table students with columns id, name, and score. You want to select students who scored between 70 and 90 inclusive. Which query correctly uses the WHERE clause to achieve this?

hard

A. SELECT * FROM students WHERE score > 70 AND score < 90;

B. SELECT * FROM students WHERE score > 70 OR score < 90;

C. SELECT * FROM students WHERE score BETWEEN 70 AND 90;

D. SELECT * FROM students WHERE score = 70 AND score = 90;

Solution

1. Step 1: Understand the condition for scores between 70 and 90 inclusive

   We want scores greater than or equal to 70 and less than or equal to 90.

2. Step 2: Evaluate each option

   SELECT * FROM students WHERE score > 70 AND score < 90; uses > and < which excludes the boundary values 70 and 90, incorrect. SELECT * FROM students WHERE score > 70 OR score < 90; uses OR which is incorrect because it includes scores outside the range. SELECT * FROM students WHERE score BETWEEN 70 AND 90; uses BETWEEN which is a concise way to express the inclusive range. SELECT * FROM students WHERE score = 70 AND score = 90; checks if score equals both 70 and 90 simultaneously, which is impossible.

3. Step 3: Choose the best correct option

   BETWEEN is the simplest and most readable way to filter inclusive ranges.

4. Final Answer:

   SELECT * FROM students WHERE score BETWEEN 70 AND 90; -> Option C

5. Quick Check:

   BETWEEN includes boundaries, perfect for range filtering [OK]

Hint: Use BETWEEN for inclusive ranges, AND for combined conditions [OK]

Common Mistakes:

• Using OR instead of AND for range conditions
• Confusing BETWEEN boundaries as exclusive
• Using impossible conditions with AND equals