Distinct deduplication

DISTINCT Overview

SELECT DISTINCT filters duplicate rows in the result set, ensuring unique values for specified columns or combined‑column sets.

Key point: When deduplicating multiple columns, uniqueness applies to the full combination of all columns in a row, not to each column separately.

Syntax

Deduplicate Single Column

SELECT DISTINCT
    column1
FROM
    tablename;

Deduplicate Multiple Combined Columns

SELECT DISTINCT column1,column2,...
FROM
    tablename;

Practical Examples

Query sex column without deduplication

SQL> select sex from student;

SEX
----
Male
Male
FemaleCode language: JavaScript (javascript)

DISTINCT for single‑column deduplication

SQL> select distinct sex from student;

SEX
----
Male
FemaleCode language: JavaScript (javascript)

Multi‑column deduplication (id + sex)

No rows get filtered because every combination of id and sex is unique.

SQL> select distinct id, sex from student;

        ID SEX
---------- ----
         1 Male
         2 Male
         3 FemaleCode language: JavaScript (javascript)

Distinct deduplication