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When an index is declared unique, multiple table rows with equal indexed values will not be allowed. Nulls are not considered equal. A multicolumn unique index will only reject cases where all of the indexed columns are equal in two rows. This is consistent with the SQL:2003-defined behavior of scalar Null comparisons.
[NOT] IN: Equal to one of multiple possible values DeptCode IN (101, 103, 209) IS [NOT] NULL: Compare to null (missing data) Address IS NOT NULL: IS [NOT] TRUE or IS [NOT] FALSE: Boolean truth value test PaidVacation IS TRUE: IS NOT DISTINCT FROM: Is equal to value or both are nulls (missing data) Debt IS NOT DISTINCT FROM-Receivables: AS: Used ...
Common Lisp has multiple other sets of equality and relational operators serving different purposes, including eq, eql, equal, equalp, and string=. [6] Older Lisps used equal , greaterp , and lessp ; and negated them using not for the remaining operators.
SQL was initially developed at IBM by Donald D. Chamberlin and Raymond F. Boyce after learning about the relational model from Edgar F. Codd [12] in the early 1970s. [13] This version, initially called SEQUEL (Structured English Query Language), was designed to manipulate and retrieve data stored in IBM's original quasirelational database management system, System R, which a group at IBM San ...
When dealing with columnar value sets, there are three types of cardinality: high-cardinality, normal-cardinality, and low-cardinality. High-cardinality refers to columns with values that are very uncommon or unique. High-cardinality column values are typically identification numbers, email addresses, or user names.
In SQL, which uses a three-valued logic for explicit comparisons because of its special treatment of Nulls, the Boolean data type (introduced in SQL:1999) is also defined to include more than two truth values, so that SQL Booleans can store all logical values resulting from the evaluation of predicates in SQL.
In addition to basic equality and inequality conditions, SQL allows for more complex conditional logic through constructs such as CASE, COALESCE, and NULLIF.The CASE expression, for example, enables SQL to perform conditional branching within queries, providing a mechanism to return different values based on evaluated conditions.
Set operations in SQL is a type of operations which allow the results of multiple queries to be combined into a single result set. [ 1 ] Set operators in SQL include UNION , INTERSECT , and EXCEPT , which mathematically correspond to the concepts of union , intersection and set difference .