Precedence & Parentheses

As soon as one WHERE has both AND and OR, the question becomes: which part is grouped with which? SQL does not read left-to-right — it follows operator precedence, just like maths does × before +. Getting this wrong is one of the most common bugs in real queries.

Definition

Operator precedence is the fixed order in which SQL evaluates operators in an expression when no parentheses are present. For conditions the order is: comparison operators (=, <>, >, …) first, then NOT, then AND, then OR last. Parentheses ( ) override this order — whatever is inside them is evaluated first.

Precedence table

So A OR B AND C is read as A OR (B AND C) — exactly like a + b × c is a + (b × c).

Syntax

SELECT column_list
FROM table_name
WHERE (condition1 OR condition2)
  AND condition3;

Example — the bug: no parentheses

Goal: "MI or RCB players who scored more than 600".

SELECT name, team, country, runs
FROM players
WHERE team = 'MI' OR team = 'RCB' AND runs > 600;

Rohit (500) and Bumrah (20) sneak in! Because AND runs before OR, SQL read it as:

team = 'MI' OR (team = 'RCB' AND runs > 600) → every MI player, plus RCB players over 600.

Example — the fix: add parentheses

SELECT name, team, country, runs
FROM players
WHERE (team = 'MI' OR team = 'RCB')
  AND runs > 600;

Now the team test is evaluated as a group first, and then every row must also have more than 600 runs.

Example — same words, different groups

-- A: Indian, and (MI or 700+)
SELECT name, team, runs
FROM players
WHERE country = 'India'
  AND (team = 'MI' OR runs > 700);

-- B: (Indian and MI), or 700+
SELECT name, team, runs
FROM players
WHERE country = 'India' AND team = 'MI'
   OR runs > 700;

Query B lets Jos Buttler (England) in, because runs > 700 stands on its own side of the OR.

Common mistakes

• ❌ Relying on memory of precedence in mixed AND/OR conditions. ✅ Always put parentheses around OR groups when AND is also present — it costs nothing and makes intent clear.
• ❌ Indenting to "show" grouping: indentation means nothing to SQL. Only parentheses do.
• ❌ Unbalanced brackets: WHERE (team = 'MI' OR team = 'RCB' AND runs > 600 → syntax error.

Tip: Often the cleanest fix is IN: WHERE team IN ('MI', 'RCB') AND runs > 600 — no OR, no precedence trap.

Interview tip

Q: What does WHERE a = 1 OR b = 2 AND c = 3 mean? A: a = 1 OR (b = 2 AND c = 3), because AND has higher precedence than OR. If you meant (a = 1 OR b = 2) AND c = 3, you must write the parentheses.