Search results
Results From The WOW.Com Content Network
Join order: Because it joins functions commutatively and associatively, the order in which the system joins tables does not change the final result set of the query. However, join-order could have an enormous impact on the cost of the join operation, so choosing the best join order becomes very important.
Two possible query plans for the triangle query R(A, B) ⋈ S(B, C) ⋈ T(A, C); the first joins S and T first and joins the result with R, the second joins R and S first and joins the result with T Relational database management systems often include a query optimizer which attempts to determine the most efficient way to execute a given query.
Queries can be nested so that the results of one query can be used in another query via a relational operator or aggregation function. A nested query is also known as a subquery. While joins and other table operations provide computationally superior (i.e. faster) alternatives in many cases, the use of subqueries introduces a hierarchy in ...
The command object can send SQL queries directly to the provider through the use of the CommandText property, send a parameterised query or stored procedure through the use of a Parameter object or Parameters collection or run a query and return the results to a dataset object via the Execute method. There are several other methods that can be ...
Queries can be nested so that the results of one query can be used in another query via a relational operator or aggregation function. A nested query is also known as a subquery . While joins and other table operations provide computationally superior (i.e. faster) alternatives in many cases (all depending on implementation), the use of ...
SQL outer joins, including left outer joins, right outer joins, and full outer joins, automatically produce Nulls as placeholders for missing values in related tables. For left outer joins, for instance, Nulls are produced in place of rows missing from the table appearing on the right-hand side of the LEFT OUTER JOIN operator. The following ...
To discover such errors, one must perform a query that uses a left outer join between the table with the foreign key and the table with the primary key, showing both key fields in addition to any fields required to distinguish the record; all invalid foreign-key values will have the primary-key column as NULL. The need to perform such a check ...
A right join is employed over the Target (the INTO table) and the Source (the USING table / view / sub-query)--where Target is the left table and Source is the right one. The four possible combinations yield these rules: