What is semi join in SQL?

A semi-join returns rows from one table that would join with another table without performing a complete join. It doesn't have explicit syntax.

Eg : semi join to evaluate an exists sub query

select *

from Customers C

where exists (

select *

from Sales S

where S.Cust_Id = C.Cust_Id

)

Cust_Id Cust_Name

----------- ----------

2 John Doe

3 Jane Doe