Database - Introduction to SQL - Discussion

Unique is constraint but distinct is not a constraint.

"Specify DISTINCT or UNIQUE if you want Oracle to return only one copy of each set of duplicate rows selected (these two keywords are synonymous). Duplicate rows are those with matching values for each expression in the select list.

So, there should be no difference in the two (performance or otherwise) as they are treated the same by Oracle.

On another note, don't try to use UNIQUE in SELECT statements for other databases (MySQL, PostgreSQL, etc). You get up with an error because ANSI standards use DISTINCT with SELECT statements. Using UNIQUE with SELECT statements is just a product-specific remnant of older Oracle DBs created before the ANSI standard was born.

Select unique * from emp, it is also showed unique data.

Unique is for primary key, distinct removes duplicity in data.

Really unique and distinct keywords are same?

What about UNIQUE keyword ?

Both unique and distinct are same ?

Oracle official documentation:

http://download.oracle.com/docs/cd/B28359_01/server.111/b28286/statements_10002.htm#sthref9346

States that:

DISTINCT | UNIQUE

Specify DISTINCT or UNIQUE if you want the database to return only one copy of each set of duplicate rows selected. These two keywords are synonymous. Duplicate rows are those with matching values for each expression in the select list.

Yes. Because Its Keyword .

SELECT DISTINCT column_name(s)
FROM table_name

P_Id	LastName	FirstName     Address	City
1	Hansen		Timoteivn      10       Sandnes
2	Svendson	Borgvn         23	Sandnes
3	Pettersen	Storgt         20	Stavanger

SELECT DISTINCT City FROM Persons

The result-set will look like this:

City
Sandnes
Stavanger

I think its Right.

Distinct remove the duplicate rows from the row.