Despite the existence of the standard, SQL code is not completely portable among different database systems without adjustments.
[2] Queries allow the user to describe desired data, leaving the database management system (DBMS) to carry out planning, optimizing, and performing the physical operations necessary to produce that result as it chooses.
The query retrieves all rows from the Book table in which the price column contains a value greater than 100.00.
The asterisk (*) in the select list indicates that all columns of the Book table should be included in the result set.
Example output might resemble the following: Under the precondition that isbn is the only common column name of the two tables and that a column named title only exists in the Book table, one could re-write the query above in the following form: However, many[quantify] vendors either do not support this approach, or require certain column-naming conventions for natural joins to work effectively.
While joins and other table operations provide computationally superior (i.e. faster) alternatives in many cases, the use of subqueries introduces a hierarchy in execution that can be useful or necessary.
CTEs can also be recursive by referring to themselves; the resulting mechanism allows tree or graph traversals (when represented as relations), and more generally fixpoint computations.
As a result, the derived table provides the result set with additional columns (the number of items sold and the company that sold the books): The concept of Null allows SQL to deal with missing information in the relational model.
Comparisons with Null, for instance equality (=) in WHERE clauses, results in an Unknown truth value.
Along with True and False, the Unknown resulting from direct comparisons with Null thus brings a fragment of three-valued logic to SQL.
This is in line with the interpretation that Null does not have a value (and is not a member of any data domain) but is rather a placeholder or "mark" for missing information.
SQL:1999 also introduced BOOLEAN type variables, which according to the standard can also hold Unknown values if it is nullable.
The following example shows a classic transfer of funds transaction, where money is removed from one account and added to another.
The precision is a positive integer that determines the number of significant digits in a particular radix (binary or decimal).
The current system date / time of the database server can be called by using functions like CURRENT_DATE, CURRENT_TIMESTAMP, LOCALTIME, or LOCALTIMESTAMP.
(Popular vendor specific functions are TO_DATE, TO_TIME, TO_TIMESTAMP, YEAR, MONTH, DAY, HOUR, MINUTE, SECOND, DAYOFYEAR, DAYOFMONTH and DAYOFWEEK.)