WHERE clause in SQL specifies that a SQL Data Manipulation Language (DML) statement should only affect rows that meet specified criteria. The criteria are expressed in the form of predicates.
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.
WHERE is an SQL reserved word.
WHERE clause is used in conjunction with SQL DML statements, and takes the following general form:
SQL-DML-Statement FROM table_name WHERE predicate
all rows for which the predicate in the
WHERE clause is True are affected (or returned) by the SQL DML statement or query. Rows for which the predicate evaluates to False or Unknown (NULL) are unaffected by the DML statement or query.
The following query returns only those rows from table mytable where the value in column mycol is greater than 100.
SELECT * FROM mytable WHERE mycol > 100
DELETE statement removes only those rows from table mytable where the column mycol is either NULL or has a value that is equal to 100.
DELETE FROM mytable WHERE mycol IS NULL OR mycol = 100
Simple predicates use one of the operators
IS NULL or
IS NOT NULL.
Predicates can be enclosed in parentheses if desired. The keywords
OR can be used to combine two predicates into a new one. If multiple combinations are applied, parentheses can be used to group combinations to indicate the order of evaluation. Without parentheses, the
AND operator has a stronger binding than
The following example deletes rows from mytable where the value of mycol is greater than 100, and the value of item is equal to the string literal 'Hammer':
DELETE FROM mytable WHERE mycol > 100 AND item = 'Hammer'
IN will find any values existing in a set of candidates.
SELECT ename WHERE ename IN ('Montreal', 'Quebec')
All rows match the predicate if their value is one of the candidate set of values. This is the same behavior as
SELECT ename WHERE ename='value1' OR ename='value2'
except that the latter could allow comparison of several columns, which each
IN clause does not. For a larger number of candidates,
IN is less verbose.
BETWEEN will find any values within a range.
SELECT ename WHERE ename BETWEEN 'value1' AND 'value2'
SELECT salary from emp WHERE salary BETWEEN 5000 AND 10000
All rows match the predicate if their value is between 'value1' and 'value2', inclusive.
LIKE will find a string fitting a certain description.
SELECT ename FROM emp WHERE ename LIKE 'S%';
SELECT ename FROM emp WHERE ename LIKE '%S';
SELECT ename FROM emp WHERE ename LIKE '%S%';
SELECT ename FROM emp WHERE ename LIKE '%A_E%';
SELECT ename FROM emp WHERE ename LIKE '[a-zA-Z0-9_]%';
The LIKE predicate typically performs a search without the normal performance benefit of indexes. Using '=', '<>', etc.. instead will increase performance. Case sensitivity (e.g., 'S' versus 's') may be different based upon database product or configuration.
Edited: 2021-06-19 17:54:38