Boolean column filtering patterns in PostgreSQL - Step-by-Step Execution

Concept Flow - Boolean column filtering patterns

Start with table

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Apply WHERE clause

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Check Boolean column value

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Include row

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Return filtered rows

The query filters rows by checking the Boolean column in the WHERE clause, including only rows where the condition is TRUE.

Execution Sample

PostgreSQL

SELECT * FROM tasks WHERE completed = TRUE;

This query returns all rows from the tasks table where the completed column is TRUE.

Execution Table

Step	Row ID	completed value	Condition (completed = TRUE)	Include in Result
1	1	TRUE	TRUE = TRUE	Yes
2	2	FALSE	FALSE = TRUE	No
3	3	TRUE	TRUE = TRUE	Yes
4	4	NULL	NULL = TRUE (unknown)	No
5	5	FALSE	FALSE = TRUE	No
6	6	TRUE	TRUE = TRUE	Yes
Exit	-	-	No more rows	Filtering complete

💡 All rows checked; only rows with completed = TRUE included.

Variable Tracker

Variable	Start	After Row 1	After Row 2	After Row 3	After Row 4	After Row 5	After Row 6	Final
completed	N/A	TRUE	FALSE	TRUE	NULL	FALSE	TRUE	N/A
Condition Result	N/A	TRUE	FALSE	TRUE	UNKNOWN	FALSE	TRUE	N/A
Included in Result	N/A	Yes	No	Yes	No	No	Yes	N/A

Key Moments - 2 Insights

Why are rows with NULL in the Boolean column not included?

Because in SQL, NULL means unknown, and the condition 'completed = TRUE' evaluates to unknown, which is treated as false in WHERE filtering (see execution_table row 4).

What happens if we write 'WHERE completed' instead of 'WHERE completed = TRUE'?

Both are equivalent in PostgreSQL; the condition checks if completed is true. The execution flow and results remain the same.

Visual Quiz - 3 Questions

Look at the execution_table, at step 2, what is the condition result for completed = TRUE?

A TRUE

B FALSE

C UNKNOWN

D NULL

💡 Hint

Check the 'Condition (completed = TRUE)' column at step 2 in the execution_table.

At which step does the condition evaluate to UNKNOWN?

A Step 4

B Step 3

C Step 5

D Step 6

💡 Hint

Look for 'NULL = TRUE (unknown)' in the Condition column in the execution_table.

If we change the query to 'WHERE completed = FALSE', which row(s) would be included?

A Rows 4 and 5

B Rows 1, 3, and 6

C Rows 2 and 5

D Rows 1 and 6

💡 Hint

Refer to the 'completed value' column and think which rows have FALSE.

Concept Snapshot

Boolean column filtering in SQL:
Use WHERE column = TRUE to select rows where the Boolean column is true.
Rows with FALSE or NULL are excluded.
NULL means unknown and does not pass the filter.
'WHERE column' is shorthand for 'WHERE column = TRUE'.

Full Transcript

This visual execution trace shows how a SQL query filters rows based on a Boolean column named 'completed'. The query selects rows where completed is TRUE. Each row's completed value is checked against TRUE. Rows with TRUE pass the condition and are included in the result. Rows with FALSE or NULL do not pass and are excluded. NULL is treated as unknown and fails the filter. The trace highlights that 'WHERE completed' is equivalent to 'WHERE completed = TRUE' in PostgreSQL. This helps beginners understand how Boolean filtering works in SQL and why NULL values behave differently.