Search results
Results from the WOW.Com Content Network
The SQL COALESCE function or CASE expressions can be used to "simulate" Null equality in join criteria, and the IS NULL and IS NOT NULL predicates can be used in the join criteria as well. The following predicate tests for equality of the values A and B and treats Nulls as being equal.
A left outer join returns all the values from an inner join plus all values in the left table that do not match to the right table, including rows with NULL (empty) values in the link column.
2.2 Microsoft SQL Server. 2.3 MySQL. ... in order to avoid the insertion of a new row with null values in all ... StartDate FROM HistoryTable AS H2 LEFT OUTER JOIN ...
Outer joins are not considered part of the classical relational algebra discussed so far. [5] The operators defined in this section assume the existence of a null value, ω, which we do not define, to be used for the fill values; in practice this corresponds to the NULL in SQL.
The join operation defined for relational databases is often referred to as a natural join (⋈). In this type of join, two relations are connected by their common attributes. MySQL's approximation of a natural join is the Inner join operator. In SQL, an INNER JOIN prevents a cartesian product from occurring when there are two tables in a query.
Additionally there is a single-row version, UPDATE OR INSERT INTO tablename (columns) VALUES (values) [MATCHING (columns)], but the latter does not give you the option to take different actions on insert versus update (e.g. setting a new sequence value only for new rows, not for existing ones.)
Microsoft SQL Server Management Studio displaying a sample query plan. The Microsoft SQL Server Management Studio tool, which ships with Microsoft SQL Server, for example, shows this graphical plan when executing this two-table join example against an included sample database:
The hash join is an example of a join algorithm and is used in the implementation of a relational database management system.All variants of hash join algorithms involve building hash tables from the tuples of one or both of the joined relations, and subsequently probing those tables so that only tuples with the same hash code need to be compared for equality in equijoins.