Operator precedence in WHERE in SQL - Step-by-Step Execution

Concept Flow - Operator precedence in WHERE

Start: Evaluate WHERE clause

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Evaluate NOT operators

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Evaluate AND operators

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Evaluate OR operators

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Filter rows based on final condition

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Output rows

The WHERE clause conditions are evaluated following operator precedence: NOT first, then AND, then OR, to decide which rows to keep.

Execution Sample

SQL

SELECT * FROM employees
WHERE NOT department = 'Sales' AND salary > 50000 OR age < 30;

This query filters employees by combining NOT, AND, and OR conditions with precedence rules.

Execution Table

Step	Condition Evaluated	Evaluation Result	Explanation
1	NOT department = 'Sales'	TRUE or FALSE per row	NOT flips TRUE/FALSE of department='Sales'
2	NOT department = 'Sales' AND salary > 50000	TRUE or FALSE per row	AND requires both sides TRUE
3	(NOT department = 'Sales' AND salary > 50000) OR age < 30	TRUE or FALSE per row	OR requires either side TRUE
4	Filter rows where final condition is TRUE	Rows kept	Only rows passing final condition remain

💡 All rows evaluated; rows not meeting final condition are excluded

Variable Tracker

Row	department='Sales'	NOT department='Sales'	salary>50000	AND result	age<30	Final condition
1	TRUE	FALSE	TRUE	FALSE	FALSE	FALSE
2	FALSE	TRUE	TRUE	TRUE	FALSE	TRUE
3	FALSE	TRUE	FALSE	FALSE	TRUE	TRUE
4	TRUE	FALSE	FALSE	FALSE	TRUE	TRUE

Key Moments - 3 Insights

Why does NOT apply before AND and OR in the WHERE clause?

Because operator precedence rules say NOT is evaluated first, so in step 1 of the execution_table, NOT flips the condition before AND and OR combine results.

What happens if we ignore operator precedence and evaluate left to right?

The final filtered rows would be different because AND and OR combine conditions differently; the execution_table rows 2 and 3 show how AND and OR combine after NOT.

How does parentheses change the evaluation order?

Parentheses force evaluation inside them first, overriding precedence. Without parentheses, the order is NOT, then AND, then OR as shown in the concept_flow.

Visual Quiz - 3 Questions

Look at the variable_tracker, what is the value of 'NOT department='Sales'' for row 2?

A NULL

B TRUE

C FALSE

D Depends on salary

💡 Hint

Check the second row under 'NOT department='Sales'' column in variable_tracker

At which step in the execution_table is the AND operator evaluated?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Condition Evaluated' column for AND operator in execution_table

If we add parentheses like WHERE NOT (department = 'Sales' AND salary > 50000) OR age < 30, how does the evaluation order change?

A NOT applies to entire AND condition first

B AND applies after OR

C OR applies before NOT

D No change in order

💡 Hint

Parentheses force evaluation inside first, changing order from concept_flow

Concept Snapshot

WHERE clause evaluates conditions using operator precedence:
1. NOT is evaluated first
2. THEN AND combines conditions
3. THEN OR combines conditions
Use parentheses to override this order
This controls which rows are filtered in the query

Full Transcript

In SQL, the WHERE clause filters rows by evaluating conditions. The order of evaluation follows operator precedence: NOT is evaluated first, then AND, then OR. This means NOT flips conditions before AND combines them, and OR combines the results last. For example, in the query filtering employees, NOT department='Sales' is evaluated first for each row. Then AND combines that with salary>50000. Finally, OR includes rows where age<30. Parentheses can change this order by forcing evaluation inside them first. Understanding this order helps predict which rows the query returns.