Tutorial by Topics: union

UNION keyword in SQL is used to combine to SELECT statement results with out any duplicate. In order to use UNION and combine results both SELECT statement should have same number of column with same data type in same order, but the length of column can be different. SELECT column_1 [, column_2 ]...

Unions are very useful tools, but come with a few important caveats: It is undefined behavior, per the C++ standard, to access an element of a union that was not the most recently modified member. Although a lot of C++ compilers permit this access in well defined ways, these are extensions an...

UNION DISTINCT -- dedups after combining the SELECTs UNION ALL -- non dedup (faster) UNION -- the default is DISTINCT SELECT ... UNION SELECT ... -- is OK, but ambiguous on ORDER BY ( SELECT ... ) UNION ( SELECT ... ) ORDER BY ... -- resolves the ambiguity UNION does not use multiple CP...

SELECT column_name(s) FROM table1 UNION SELECT column_name(s) FROM table2; SELECT column_name(s) FROM table1 UNION ALL SELECT column_name(s) FROM table2; SELECT column_name(s) FROM table1 WHERE col_name="XYZ" UNION ALL SELECT column_name(s) FROM table2 WHERE col_name="XYZ&qu...

Union types are used in several languages, notably C-language, to contain several different types which can "overlap" in the same memory space. In other words, they might contain different fields all of which start at the same memory offset, even when they might have different lengths an...

A union-find (or disjoint-set) data structure is a simple data structure a partition of a number of elements into disjoint sets. Every set has a representative which can be used to distinguish it from the other sets. It is used in many algorithms, e.g. to compute minimum spanning trees via Kruskal'...