Implicit value conversion

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When an operator is used with operands of different types, type conversion implicitly occurs to make the operands compatible. For example, numbers are converted to strings as necessary, and vice versa.

Note: explicit conversions (using the CAST function) are not yet available

The following table describes how conversion occurs for comparison operations:

Left	Right	Cast as	NULL evaluation
VARCHAR	INTEGER	VARCHAR	-
VARCHAR	DOUBLE	VARCHAR	-
VARCHAR	NULL	VARCHAR	$right = ''
INTEGER	VARCHAR	VARCHAR	-
INTEGER	DOUBLE	DOUBLE	-
INTEGER	NULL	INTEGER	$right = 0
DOUBLE	VARCHAR	VARCHAR	-
DOUBLE	INTEGER	DOUBLE	-
DOUBLE	NULL	DOUBLE	$right = 0.0
NULL	VARCHAR	VARCHAR	$left = ''
NULL	INTEGER	INTEGER	$left = 0
NULL	DOUBLE	DOUBLE	$left = 0.0
NULL	NULL	VARCHAR	$left = '', $right = ''

Some more notes about comparison:

· NULL = NULL, NULL = 0 and NULL = '' (and vice-versa) are always evaluated to true

· the DATE, TIME, DATETIME and TIMESTAMP data types are internally mapped to TIMESTAMP and their comparisons occur like for INTEGER types

· comparisons that use floating-point numbers (or values that are converted to floating-point numbers) are approximate because such numbers are inexact and usually have only 53 bits of precision and are subject to rounding

· the conversion from string to floating-point and from integer to floating-point do not necessarily occur the same way. The integer may be converted to floating-point by the CPU, whereas the string is converted digit by digit in an operation that involves floating- point multiplications.