ORDER BY clause in SQL specifies that a SQL
SELECT statement returns a result set with the rows being sorted by the values of one or more columns. The sort criteria do not have to be included in the result set. The sort criteria can be expressions, including column names, user-defined functions, arithmetic operations, or
CASE expressions. The expressions are evaluated and the results are used for the sorting, i.e., the values stored in the column or the results of the function call.
ORDER BY is the only way to sort the rows in the result set. Without this clause, the relational database system may return the rows in any order. If an ordering is required, the
ORDER BY must be provided in the
SELECT statement sent by the application. Although some database systems allow the specification of an
ORDER BY clause in subselects or view definitions, the presence there has no effect. A view is a logical relational table, and the relational model mandates that a table is a set of rows, implying no sort order whatsoever. The only exception are constructs like
ORDER BY ORDER OF ... (not standardized in SQL:2003) which allow the propagation of sort criteria through nested subselects.
The SQL standard's core functionality does not explicitly define a default sort order for Nulls. With the SQL:2003 extension T611, "Elementary OLAP operations", nulls can be sorted before or after all data values by using the
NULLS FIRST or
NULLS LAST clauses of the
ORDER BY list, respectively. Not all DBMS vendors implement this functionality, however. Vendors who do not implement this functionality may specify different treatments for Null sorting in the DBMS.
ORDER BY ... DESC will order in descending order, otherwise ascending order is used. (The latter may be specified explicitly using
SELECT * FROM Employees ORDER BY LastName, FirstName
This sorts by the LastName field, then by the FirstName field if LastName matches.
Edited: 2021-06-19 17:54:32