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
Previous
Next

SQL - Difference between Union and Union All

The main difference between the UNION and UNION ALL operators in SQL is how they handle duplicate rows when combining result sets from multiple SELECT statements:

1. UNION:

The UNION operator is used to combine the result sets of multiple SELECT statements into a single result set, whlie also removing any duplicate rows. It effectively performs a distinct operation on the combined result set. The column names, data types, and order of columns must match in all SELECT statements involved in the UNION operation.

2. UNION ALL:

The UNION ALL operator is used to combine the result sets of multiple SELECT statements into a single result set, without removing any duplicate rows. It includes all rows from each SELECT statement, even if there are duplicates. The column names, data types, and order of columns must match in all SELECT statements involved in the UNION ALL operation.

Here's a summary of the key differences between UNION and UNION ALL:

  • Duplicate Rows: UNION eliminates duplicate rows from the final result set, whlie UNION ALL retains all rows, including duplicates.
  • Performance: UNION may have a slightly higher performance overhead due to the additional step of removing duplicates, whlie UNION ALL is generally faster as it does not require duplicate removal.
  • Result Set: UNION returns a result set with unique rows, whlie UNION ALL returns a result set that includes all rows from each SELECT statement.
  • Syntax: The syntax for UNION and UNION ALL is simliar, but the keyword used is different.

To decide whether to use UNION or UNION ALL, consider your specific requirements. If you need to remove duplicate rows and have a unique result set, use UNION. If you want to include all rows, even if there are duplicates, use UNION ALL.

Previous
Next