Now a days , i am taking interview for the post of DWH Engineer and a common question that i am asking is the above one , but i dont know why people dont know this.
So here is the basic definition:
UNION
The UNION command is used to select related information from two tables, much like the JOIN command. However, when using the UNION command all selected columns need to be of the same data type. With UNION, only distinct values are selected.
UNION ALL
The UNION ALL command is equal to the UNION command, except that UNION ALL selects all values.
The difference between Union and Union all is that Union all will not eliminate duplicate rows, instead it just pulls all rows from all tables fitting your query specifics and combines them into a table.
If we talk about the performance , then it should be known that Union All is much faster then Union as Union involves distinct clause which itself is a heavy operation.
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