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What is difference between UNION and UNION ALL in SQL Server

We use UNION and UNION ALL operator to combine multiple results set into one result set.
UNION operator is used to combining multiple results set into one result set but removes any duplicate rows. Basically, UNION is used to performing a DISTINCT operation across all columns in the result set. UNION operator has the extra overhead of removing duplicate rows and sorting result.
UNION ALL operator use to combine multiple results set into one result set but it does not remove any duplicate result. Actually, this does not remove duplicate rows so it is faster than the UNION operator. If you want to combine multiple results and without duplicate records then use UNION otherwise UNION ALL is better.
Following some rules for using UNION/UNION ALL operator
1.The number of the column should be the same in the query's when you want to combine them. 2.The column should be of the same data type. 3.ORDER BY clause can be applied to the overall result set not within each result set.
4.Column name of …

Difference between VARCHAR and NVARCHAR in SQL Server

1.VARCHAR is Non-Unicode variable length character data type while NVARCHAR Unicode variable length character data type.
2.VARCHAR takes 1 byte per character while NVARCHAR takes 2 byte per Unicode/Non-Unicode character. For example:

DECLARE @name ASVARCHAR(50)='Dilip' SELECT @name AS Name, DATALENGTH(@name)AS [Length]

Result Set     Name                                               Length     -------------------------------------------------- -----------     Dilip                                              5
    (1 row(s) affected)

DECLARE @name ASNVARCHAR(50)='Dilip' SELECT @name AS Name, DATALENGTH(@name)AS [Length]

Result Set Name                                               Length

What is Fill Factor in SQL Server