The WHERE clause is a part of the SELECT, UPDATE and DELETE DML statements where it determines a subset of records the DML statements will work with.
Syntax of the WHERE clause
WHERE {condition}
Condition is equal to:
[(]attr_name [NOT] <|<=|=|=>|>|IS[ NOT ]NULL|IN|LIKE|BETWEEN value1 [AND value2] [ESCAPE esc_mark]][({values})] [)][ AND|OR] [ condition]
Comments
The keyword WHERE is followed by conditions, one condition for one attribute, joined together with taken of AND, OR and round brackets.
| attr_name | The name of an attribute that's value is to be used. |
| NOT | Negates the rest of an expression. |
| <, <=, =, =>, > | The attribute's value is one of following: less, less or equal, equal, equal or greater, greater than value1. |
| IS NULL, IS NOT NULL | Tests the attribute's value for the null one |
| IN ({values}) | Tests the attribute's value if is it one of {values} value. |
| LIKE | Tests the attribute's value if does it begin with a string represented by value1. |
| ESCAPE esc_mark | In the case when of searching for strings beginning with
value1, there must be value1%. The '%' character is called the escape mark. As there may
be the need to search for a string containing this character, the ESCAPE gives the way to
define new escape mark in the scope of the entire statement. As the '%' character means all characters after, the '_' escape mark means only one character after. In the case of a redefined escape mark, the value must begin and end with '%'. The redefined escape mark must be followed either by the '%' or '_'. |
| BETWEEN | Tests the attribute's value if is it between value1 and value2. |