Question

Today i found a query like

SELECT 1 [Col_name] FROM MyTable

and

SELECT  [Col_name] FROM MyTable

Both seems to return the same result. I am confused.

This is the actual query:

SELECT 1 Col1 FROM [Table1] WHERE Col1 = 1 

UNION

SELECT 2 Col1  FROM [Table1] 

Any help is appreciated

Answer 1

Your query:

SELECT 1 [Col_name] FROM MyTable

will return the literal value 1 with an alias col_name. Even if col_name is an identifier in your table.

However:

SELECT [Col_name] FROM MyTable;

will select col_name from your table.

The same with:

SELECT 1 Col1 FROM [Table1] WHERE Col1 = 1 
UNION
SELECT 2 Col1  FROM [Table1] 

Will give you only two rows:

1
2

regardless the values in the table. Because SELECT 1 Col1 FROM [Table1] WHERE Col1 = 1 returns the literal value 1 with an alias col1, SELECT 2 Col1 FROM [Table1] returns the literal value 2 with the same alias col1 with UNION(Distinct select) set operator, will give you only two values (1, 2) since they are the only distinct values.

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