SQL Server BasicsWhat is SQL Server database?What is RDBMS?What is Normalization?Why we use Denormalization?What_is_SQL?What is PL/SQL?Difference between SQL and PL/SQLDatabase TableOne to One RelationshipOne to Many RelationshipMany to Many RelationshipMany to One RelationshipString Data TypesNumber Data TypesDate Data TypesOther Data TypesCreate DatabaseDrop DatabaseCreating and Managing Users in SQL ServerCreate TableAlter TableDrop TableConstraints in SQL serverPrimary KeyForeign KeyUnique KeyCandidate KeyComposite KeyDifference between primary key and candidate keyPrimary key and foreign key relationshipSurrogate KeyCascading Referential Integrity ConstraintsSelf Referential Integrity ConstraintsInsert into statementInsert multiple recordsUpdate statementDelete statementTruncate statementDifference between Delete and TruncateAlias in SQL ServerSelect statementSelect DistinctSelect TopSelect IntoNull Functions(ISNULL(),NULLIF(),COALESCE())Sub QueryIdentity ColumnSequence objectDifference between sequence and identity columnSQL Server ClausesWHERE ClauseOrder By ClauseTop N ClauseGroup By ClauseHaving ClauseDifference between Where and HavingSQL Server OperatorsArithmetic OperatorsComparison OperatorsLogical OperatorsBitwise OperatorsAny OperatorsAll OperatorsUnion OperatorsUnion All OperatorsDifference between Union and Union AllIntersect OperatorExcept OperatorDifference between Except and IntersectJoinsInner JoinLeft JoinRight JoinFull JoinSelf JoinCross JoinViewsWhat are views?Create views using SSMSIndexed ViewsComplex ViewsCheck Option in ViewCheck Encryption in ViewSchema Binding Option in ViewRead-only ViewsUpdatable ViewsAdvantages and disadvantages of viewsCreate multiple views on one tableCan we implement index on views?Can we Perform Insert, update, delete operation on views?Stored Procedure and FunctionsWhat are Stored Procedures?Why we use stored procedures?Passing parameters to Stored procedureUser-Defined FunctionsDifference between UDF and Stored procedurePre-Defined Functions@@Indentity and Scope_IndentityNULLIF, ISNULL and COALESCE

SQL - Union Operator

In SQL Server, the UNION operator is used to combine the result sets of two or more SELECT statements into a single result set. The UNION operator removes duplicate rows from the final result set by default.

The basic syntax for using the UNION operator in SQL Server is as follows:


SELECT column1, column2, ...
FROM table1
UNION
SELECT column1, column2, ...
FROM table2;

In this syntax, column1, column2, etc., represent the columns you want to select from the respective tables (table1, table2, etc.). The SELECT statements can include WHERE, ORDER BY, and other clauses as needed.

The UNION operator combines the result sets of the two SELECT statements and returns a single result set with the combined rows. The columns in the SELECT statements must have the same data types and be in the same order.

By default, the UNION operator removes duplicate rows from the final result set.

Here's an example to demonstrate the usage of the UNION operator in SQL Server:

Consider two tables, "Employees" and "Customers", with the following structures:

Table: Employees
ID Name Department
1 John HR
2 Jane Sales
3 Mike IT
Table: Customers
ID Name Location
1 Lisa New York
2 Tom London
3 Sarah Paris

To combine the result sets of both tables while eliminating duplicates, you can use the UNION operator as follows:


SELECT ID, Name, Department
FROM Employees
UNION
SELECT ID, Name, Location
FROM Customers;

The above query will merge the result sets of the two SELECT statements and return a single result set, eliminating any duplicate rows. Here's the expected output:

ID Name Department
1 John HR
2 Jane Sales
3 Mike IT
4 Tom london
5 Sarah Paris

As you can see, the result set includes all distinct rows from both tables, combining the rows from the "Employees" table and the "Customers" table. The UNION operator removes any duplicate rows from the final result set.

It's important to note that for the UNION operator to work correctly, the column names, data types, and order must match in both SELECT statements.

In summary, the UNION operator is used to combine and eliminate duplicate rows from the result sets of multiple SELECT statements.