Conditions

Data Types and SubtypesData Definition (DDL) Statements

Conditions

A condition — or Boolean expression — is a statement about the data that, like a predicate, can resolve to TRUE, FALSE or NULL.Conditions consist of one or more predicates, possibly negated using NOT and connected by AND and OR operators.Parentheses may be used for grouping predicates and controlling evaluation order.

A predicate may embed other predicates.Evaluation sequence is in the outward direction, i.e. the innermost predicates are evaluated first.Each “level” is evaluated in precedence order until the truth value of the ultimate condition is resolved.

Comparison Predicates

A comparison predicate consists of two expressions connected with a comparison operator.There are six traditional comparison operators:

=, >, <, >=, <=, <>

For the complete list of comparison operators with their variant forms, see Comparison Operators.

If one of the sides (left or right) of a comparison predicate has NULL in it, the value of the predicate will be UNKNOWN.

Examples

Retrieve information about computers with the CPU frequency not less than 500 MHz and the price lower than $800:
```
SELECT *
FROM Pc
WHERE speed >= 500 AND price < 800;
```
Retrieve information about all dot matrix printers that cost less than $300:
```
SELECT *
FROM Printer
WHERE ptrtype = 'matrix' AND price < 300;
```
The following query will return no data, even if there are printers with no type specified for them, because a predicate that compares NULL with NULL returns NULL:
```
SELECT *
FROM Printer
WHERE ptrtype = NULL AND price < 300;
```
On the other hand, ptrtype can be tested for NULL and return a result: it is just that it is not a comparison test:
```
SELECT *
FROM Printer
WHERE ptrtype IS NULL AND price < 300;
```
— see [fblangref50-commons-isnotnull].

Note	Note about String Comparison When `CHAR` and `VARCHAR` fields are compared for equality, trailing spaces are ignored in all cases.