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.