What is SQL Injection attack and how to prevent it?

Union operator will return unique records from tables and Union All in SQL server will return all the records from the tables including duplicate values also.

SQL Union Operator is used to combine the result of two or more select statement queries into single result set. The Union Operator is used to select only distinct values from two tables.

SELECT column1,column2 FROM table1

UNION

SELECT column1,column2 FROM table2

Here one more thing we need to remember that is we can use Union Operator for the tables which is having same column names and same data types otherwise it will throw error .

If we want all the records then we need to use UNION ALL Operator.

SELECT column1,column2 FROM table1

UNION ALL

SELECT column1,column2 FROM table2

The main difference between Union and Union ALL operator is

Union operator will return distinct values but Union ALL returns all the values including duplicate values.

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