Boolean type behavior in PostgreSQL - Step-by-Step Execution

Concept Flow - Boolean type behavior

Input Boolean Value

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Evaluate Boolean Expression

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Result is TRUE, FALSE, or NULL

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Use Result in Query Condition or Output

Boolean values in PostgreSQL are evaluated as TRUE, FALSE, or NULL and control query logic or output accordingly.

Execution Sample

PostgreSQL

SELECT TRUE AS val_true, FALSE AS val_false, NOT TRUE AS not_true;

This query shows basic Boolean values and the NOT operator result.

Execution Table

Step	Expression	Evaluation	Result
1	TRUE	Boolean literal TRUE	true
2	FALSE	Boolean literal FALSE	false
3	NOT TRUE	Negation of TRUE	false
4	End of evaluation	All expressions evaluated	Query returns one row with three columns

💡 All Boolean expressions evaluated and returned as query output.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
val_true	undefined	true	true	true	true
val_false	undefined	undefined	false	false	false
not_true	undefined	undefined	undefined	false	false

Key Moments - 2 Insights

Why does NOT TRUE become false instead of true?

NOT reverses the Boolean value. Since TRUE is true, NOT TRUE becomes false as shown in step 3 of the execution_table.

Can Boolean values be NULL in PostgreSQL?

Yes, Boolean can be NULL representing unknown. This is different from TRUE or FALSE and affects conditions in queries.

Visual Quiz - 3 Questions

Look at the execution_table, what is the result of NOT TRUE at step 3?

A true

B false

C NULL

D Error

💡 Hint

Check the 'Result' column for step 3 in the execution_table.

At which step does the query return all Boolean values?

A Step 1

B Step 3

C Step 4

D Step 2

💡 Hint

Look for the step where the note says 'Query returns one row with three columns'.

If we add NOT FALSE to the query, what would be the result?

A true

B false

C NULL

D Error

💡 Hint

NOT reverses the Boolean value; FALSE becomes true.

Concept Snapshot

Boolean type in PostgreSQL stores TRUE, FALSE, or NULL.
Use TRUE and FALSE literals directly.
NOT operator reverses Boolean values.
NULL means unknown, different from FALSE.
Boolean values control query logic and conditions.

Full Transcript

This visual execution shows how PostgreSQL handles Boolean values. The query selects TRUE, FALSE, and NOT TRUE. Step by step, TRUE evaluates to true, FALSE to false, and NOT TRUE to false. The final output is a single row with these Boolean results. Key points include that NOT reverses Boolean values and that Boolean can also be NULL, representing unknown. This affects how conditions work in queries.