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Union is inefficient because it is scanned for duplicate values. If the merge does not intentionally delete duplicate rows, then use UNION ALL Two the number of SQL statement fields to be joined must be the same, and the field type should be "compatible" (consistent); If we need to display the results of the two SELECT statements as a whole, we need the union or UNION ALL keyword. The role of a Union (or un

Students who write SQL frequently may use the two Keywords Union and union all, and maybe you know that they can combine the result sets of two queries. So what's the difference? Below we have a simple analysis. Comparison Union: Distinct The two result sets, excluding duplicate rows, equivalent to the same, and ordering the default rules; UNION ALL: A set of

One: The purpose of the Union directive is to combine the results of two SQL statements.Let's say we have the following two tables:Store_information table: Store_name Sales DateLos Angeles $1500 jan-05-1999San Diego $ jan-07-1999Los Angeles jan-08-1999Boston $700 jan-08-1999Internet Sales table: Date Salesjan-07-1999jan-10-1999 $535jan-11-1999 $320jan-12-1999 $750If we want to find out all the days that have turnover (sales). To achieve this, we use t