SqlServer Questions

1. What’s the difference between a primary key and a unique key?

Both primary key and unique enforce uniqueness of the column on which they are defined. But by default primary key creates a clustered index on the column, where are unique creates a nonclustered index by default. Another major difference is that, primary key doesn’t allow NULLs, but unique key allows one NULL only.

2. Write a SQL Query to find first Week Day of month?

SELECT DATENAME(dw, DATEADD(dd, - DATEPART(dd, GETDATE()) + 1, GETDATE())) AS FirstDay

3. How to find 6th highest salary from Employee table

SELECT TOP 1 salary FROM (SELECT DISTINCT TOP 6 salary FROM employee
ORDER BY salary DESC) a ORDER BY salary

4. What is a join and List different types of joins.

Joins are used in queries to explain how different tables are related. Joins also let you select data from a table depending upon data from another table. Types of joins: INNER JOINs, OUTER JOINs, CROSS JOINs. OUTER JOINs are further classified as LEFT OUTER JOINS, RIGHT OUTER JOINS and FULL OUTER JOINS.

5. How can I enforce to use particular index?

You can use index hint (index=index_name) after the table name. SELECT au_lname FROM authors (index=aunmind)