Standard comparison operators in PostgreSQL - Step-by-Step Execution

Concept Flow - Standard comparison operators

Start Query

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Evaluate Left Operand

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Evaluate Right Operand

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Apply Comparison Operator

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Result: TRUE / FALSE / NULL

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Use Result in WHERE or Condition

The database evaluates both sides of the comparison, applies the operator, and returns TRUE, FALSE, or NULL to filter or decide query results.

Execution Sample

PostgreSQL

SELECT * FROM employees WHERE salary > 50000;

This query selects employees whose salary is greater than 50000.

Execution Table

Step	Left Operand	Operator	Right Operand	Evaluation	Result
1	salary = 45000	>	50000	45000 > 50000	FALSE
2	salary = 52000	>	50000	52000 > 50000	TRUE
3	salary = 50000	>	50000	50000 > 50000	FALSE
4	salary = NULL	>	50000	NULL > 50000	NULL
5	End of rows				Query returns rows where result is TRUE

💡 All rows evaluated; only rows with TRUE result are returned.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
salary	N/A	45000	52000	50000	NULL	N/A
comparison_result	N/A	FALSE	TRUE	FALSE	NULL	N/A

Key Moments - 2 Insights

Why does the comparison with NULL not return TRUE or FALSE?

In row 4 of the execution_table, comparing salary NULL with 50000 results in NULL because NULL means unknown, so the comparison cannot be TRUE or FALSE.

Why is salary = 50000 > 50000 FALSE and not TRUE?

In row 3, the operator '>' means strictly greater than, so 50000 > 50000 is FALSE because they are equal, not greater.

Visual Quiz - 3 Questions

Look at the execution_table, what is the result of comparing salary = 52000 with 50000 using '>'?

A TRUE

B FALSE

C NULL

D Error

💡 Hint

Check row 2 in the execution_table where salary is 52000.

At which step does the comparison result become NULL?

A Step 1

B Step 4

C Step 2

D Step 3

💡 Hint

Look for the row with salary NULL in the execution_table.

If the operator was changed from '>' to '>=' at step 3, what would be the result?

A FALSE

B NULL

C TRUE

D Error

💡 Hint

Consider that '>=' means greater than or equal, check step 3 where salary equals 50000.

Concept Snapshot

Standard comparison operators compare two values.
Common operators: =, <>, >, <, >=, <=.
Return TRUE, FALSE, or NULL (unknown).
Used in WHERE clauses to filter rows.
NULL comparisons yield NULL, not TRUE or FALSE.

Full Transcript

This visual execution shows how standard comparison operators work in PostgreSQL. The query selects employees with salary greater than 50000. Each row's salary is compared to 50000 using the '>' operator. The result is TRUE if salary is greater, FALSE if not, and NULL if salary is unknown (NULL). Only rows with TRUE results are returned. Important points include that NULL comparisons do not return TRUE or FALSE, and that '>' means strictly greater than, so equal values return FALSE.