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WHERE clauses are not mandatory clauses of SQL DML statements, but can be used to limit the number of rows affected by a SQL DML statement or returned by a query. In brief SQL WHERE clause is used to extract only those results from a SQL statement, such as: SELECT, INSERT, UPDATE, or DELETE statement. [1]
SELECT is the most complex statement in SQL, with optional keywords and clauses that include: The FROM clause, which indicates the table(s) to retrieve data from. The FROM clause can include optional JOIN subclauses to specify the rules for joining tables. The WHERE clause includes a comparison predicate, which restricts the rows returned by ...
Without an ORDER BY clause, the order of rows returned by an SQL query is undefined. The DISTINCT keyword [5] eliminates duplicate data. [6] The following example of a SELECT query returns a list of expensive books. The query retrieves all rows from the Book table in which the price column contains a value greater
HAVING and WHERE are often confused by beginners, but they serve different purposes.WHERE is taken into account at an earlier stage of a query execution, filtering the rows read from the tables.
An 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 does not have to be included in the result set (restrictions apply for SELECT DISTINCT, GROUP BY, UNION [DISTINCT], EXCEPT [DISTINCT] and INTERSECT [DISTINCT].)
(In modern data processing terms, one can think of each stage as an SQL clause: SELECT (filter columns), then WHERE (filter cards, or "rows"), then maybe a GROUP BY for totals and counts, then a SORT BY; and then perhaps feed those back to another set of SELECT and WHERE cycles again if needed.) A human operator had to retrieve, load, and store ...
From clauses are very common, and will provide the rowset to be exposed through a Select statement, the source of values in an Update statement, and the target rows to be deleted in a Delete statement. [1] FROM is an SQL reserved word in the SQL standard. [2] The FROM clause is used in conjunction with SQL statements, and takes the following ...
Recall that the Book table below has a composite key of {Title, Format}, which will not satisfy 2NF if some subset of that key is a determinant. At this point in our design the key is not finalized as the primary key , so it is called a candidate key .