## Comparisons

Values to be compared must be of compatible data types. If values with incompatible data types are compared, an error occurs.

### Character String Comparisons

Both fixed-length and variable-length character strings are compared character by character from left to right.

If the strings are of different length, the shorter string is conceptually padded to the right with blanks before the comparison is made, that is, character differences take precedence over length differences.

For example, the variable-length column with the value '`town `', one trailing blank, is equal to the variable-length column with the value '`town `', two trailing blanks.

When comparing a character string to a national character string, the character string is implicitly converted to a national character string, before the comparison is performed.

#### Collations

A collation determines whether a character string is less than or greater than another when sorting or comparing data.

SQL only permits compatible character strings to be assigned or compared. That is, you can assign or compare character strings only if the source and target strings belong to the same collation or can be coerced into having the same collation.

A character string that is defined with a named collation can only be compared or assigned to a character string that is either defined with the same named collation or is defined without a collation.

In the case where one of the strings is not associated with a named collation then it will be implicitly coerced to the same collation as the other string.

A collation specified in the column-definition will take precedence over a domain collation.

### Numerical Comparisons

Numerical values are always compared according to their algebraic values.

Integer values compared with decimal or floating point values are treated as decimal or floating respectively. When decimal values are compared with decimal, the lower precision value is conceptually padded with leading and trailing zeros as necessary. Decimal values compared with floating point values are treated as floating.

Thus all the following comparisons evaluate to true:

``` 1 = 1.0
2 < 2.3E0
35.3 = 035.300
35.3 > 3.5E1
```

### Datetime and Interval Comparisons

Two `DATETIME` values may be compared if they are assignment-compatible, as defined in Datetime Assignment Rules.

`DATETIME` comparisons are performed in accordance with chronological ordering.

When two `TIME` or two `TIMESTAMP` values are compared, the seconds precision of the value with the lowest seconds precision is extended by adding trailing zeros.

Two `INTERVAL` values may be compared if they are assignment-compatible, as defined in Interval Assignment Rules.

`INTERVAL` comparisons are performed in accordance with their sign and magnitude.

It is not possible to compare `YEAR-MONTH` intervals with `DAY-TIME` intervals.

Comparable `INTERVAL` types with different interval precisions are conceptually converted to the same interval precision, prior to any comparison, by adding fields as required.

### NULL Comparisons

All comparisons involving `NULL` on either side of the comparison operator evaluate to unknown. `NULL` is never equal to, greater than or less than anything else. SQL provides a special `NULL` predicate to test for the presence or absence of `NULL` in a column, see The NULL Predicate.

Considerable care is required in writing search conditions involving columns which may contain `NULL`. It is often very easy to overlook the effect of `NULL` comparisons, with the result that rows which should be included in the result table are omitted or vice versa. See the Mimer SQL User's Manual, Handling NULL Values, for further discussion of this point.

### Truth Tables

The following truth tables summarize the outcome of conditional expressions where comparisons are negated by `NOT` or joined by `AND` or `OR`.

A question mark (`?`) represents the truth value unknown, `T` represents the value `TRUE` and `F` represents the value `FALSE`.

T
F
F
T
?
?

T
F
?
T
T
F
?
F
F
F
F
?
?
F
?

T
F
?
T
T
T
T
F
T
F
?
?
T
?
?

T
F
?
T
T
F
F
F
F
T
F
?
F
F
T

### Standard Compliance

This section summarizes standard compliance concerning comparisons.