What is the difference between UNION and UNIONALL?
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Answer Posted / haroon nazir.s
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.
A UNION statement effectively does a SELECT DISTINCT on the
results set. If you know that all the records returned are
unique from your union, use UNION ALL instead, it gives
faster results.
Example:
Table 1 : First,Second,Third,Fourth,Fifth
Table 2 : First,Second,Fifth,Sixth
Result Set:
UNION: First,Second,Third,Fourth,Fifth,Sixth (This will
remove duplicate values)
UNION ALL:
First,First,Second,Second,Third,Fourth,Fifth,Fifth,Sixth,Sixth
(This will repeat values)
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