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In standard SQL:1999 hierarchical queries are implemented by way of recursive common table expressions (CTEs). Unlike Oracle's earlier connect-by clause, recursive CTEs were designed with fixpoint semantics from the beginning. [1] Recursive CTEs from the standard were relatively close to the existing implementation in IBM DB2 version 2. [1]
SELECT list is the list of columns or SQL expressions to be returned by the query. This is approximately the relational algebra projection operation. AS optionally provides an alias for each column or expression in the SELECT list. This is the relational algebra rename operation. FROM specifies from which table to get the data. [3]
A derived table is the use of referencing an SQL subquery in a FROM clause. Essentially, the derived table is a subquery that can be selected from or joined to. The derived table functionality allows the user to reference the subquery as a table. The derived table is sometimes referred to as an inline view or a subselect.
Common table expressions and recursive queries [ edit ] SQL:1999 added a WITH [RECURSIVE] construct allowing recursive queries, like transitive closure , to be specified in the query language itself; see common table expressions .
A common use for an expression index is to support case-insensitive searching or constraints. For example, if a web site wants to make user names case-insensitive, but still preserve the case as originally entered by the user, an index can be created on the lower-case representation of the user name:
The result of the natural join is the set of all combinations of tuples in R and S that are equal on their common ... Microsoft T-SQL and IBM DB2 do not ...
A little later, IBM also introduced Db2, another SQL-based DBMS, this one for the MVS operating system. The two products have coexisted since then; however, SQL/DS was rebranded as "DB2 for VM and VSE" in the late 1990s.
Correlated subqueries may appear elsewhere besides the WHERE clause; for example, this query uses a correlated subquery in the SELECT clause to print the entire list of employees alongside the average salary for each employee's department.