Comparison Operators in WHERE (PostgreSQL)
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We’ve already seen that the WHERE keyword allows us to filter rows returned by a query.
In simple words:
WHEREchecks each row one by one and decides whether it should appear in the final result.
Let’s now look at different comparison operators we can use inside a WHERE clause.
Recap: Filtering with WHERE
Consider this query:
select
name,
area
from cities
where area > 4000;
What happens here is very simple:
PostgreSQL looks at each row
It checks the
areavalueOnly rows with
area > 4000are returned
That’s why we only see cities like Tokyo and Shanghai.
Equality Check (=)
We can also check if a column value is exactly equal to something.
For example, let’s find the city with an area equal to 8223:
select
name,
area
from cities
where area = 8223;
Important Note
In SQL:
=is not assignmentit is a comparison operator
We are not changing anything — we are just checking a condition.
Result
This query returns:
- Tokyo
Because Tokyo is the only city with an area of 8223.
Not Equal (!=)
We can also check for values that are not equal.
select
name,
area
from cities
where area != 8223;
This returns:
- all cities except Tokyo
Alternative Not Equal Operator (<>)
PostgreSQL also supports another operator for “not equal”:
<>
So this query:
select
name,
area
from cities
where area <> 8223;
Produces the same result as !=.
Both are valid — you may see either in real projects.
How These Comparisons Work
One very important thing to remember:
The comparison in the
WHEREclause is evaluated for every single row.
PostgreSQL does this internally:
Take one row
Apply the condition
Keep or discard the row
Move to the next row
This happens until all rows are checked.
Common Comparison Operators
Here are some comparison operators we’ve seen so far:
=→ equal to!=→ not equal to<>→ not equal to>→ greater than<→ less than>=→ greater than or equal to<=→ less than or equal to
These operators form the foundation of filtering in SQL.