Simple Expressions

Logical Expressions

Logical Operators lists the operators and calculation rules of logical expressions.

Comparative Expressions

Operators lists the common comparative operators.

In addition to comparative operators, you can also use the following sentence structure:

• BETWEEN operator

  a BETWEEN x AND y is equivalent to a >= x AND a <= y.

  a NOT BETWEEN x AND y is equivalent to a < x OR a > y.

• To check whether a value is null, use:

  expression IS NULL

  expression IS NOT NULL

  or an equivalent (non-standard) sentence structure:

  expression ISNULL

  expression NOTNULL

  NOTICE: Do not write expression=NULL or expression<>(!=)NULL, because NULL represents an unknown value, and these expressions cannot determine whether two unknown values are equal.

• is distinct from/is not distinct from

  • is distinct from

    If the data types and values of A and B are different, the value is true.

    If the data types and values of A and B are the same, the value is false.

    Empty values are considered the same.

  • is not distinct from

    If the data types and values of A and B are different, the value is false.

    If the data types and values of A and B are the same, the value is true.

    Empty values are considered the same.

• <=> NULL-safe equality operator

  The comparison of NULL values is added on the basis of the comparison of '='. If neither the left nor right value of the operator is NULL, the result is the same as that of '='.

  If the data types and values of A and B are not completely the same, the value is false.

  If the data types and values of A and B are completely the same, the value is true.

  Null values are considered the same.

  NOTE:

  • The usage of the <=> operator is the same as that of IS NOT DISTINCT FROM.
  • This operator is valid only when MogDB is compatible with the MY type (that is, sql_compatibility = 'B'). Other types do not support this operator.

Pseudocolumn

ROWNUM

ROWNUM is a pseudocolumn that returns a number indicating the row number of the result obtained from the query. The value of ROWNUM in the first row is 1, the value of ROWNUM in the second row is 2, and so on.

The return type of ROWNUM is NUMERIC. ROWNUM can be used to limit the total number of rows returned by a query. For example, the following statement limits the maximum number of records returned from the table Students to 10.

select * from Students where rownum <= 10;

Examples

MogDB=# SELECT 2 BETWEEN 1 AND 3 AS RESULT;
 result
----------
 t
(1 row)

MogDB=# SELECT 2 >= 1 AND 2 <= 3 AS RESULT;
 result
----------
 t
(1 row)

MogDB=# SELECT 2 NOT BETWEEN 1 AND 3 AS RESULT;
 result
----------
 f
(1 row)

MogDB=# SELECT 2 < 1 OR 2 > 3 AS RESULT;
 result
----------
 f
(1 row)

MogDB=# SELECT 2+2 IS NULL AS RESULT;
 result
----------
 f
(1 row)

MogDB=# SELECT 2+2 IS NOT NULL AS RESULT;
 result
----------
 t
(1 row)

MogDB=# SELECT 2+2 ISNULL AS RESULT;
 result
----------
 f
(1 row)

MogDB=# SELECT 2+2 NOTNULL AS RESULT;
 result
----------
 t
(1 row)

MogDB=# SELECT 2+2 IS DISTINCT FROM NULL AS RESULT;
 result
----------
 t
(1 row)

MogDB=# SELECT 2+2 IS NOT DISTINCT FROM NULL AS RESULT;
 result
----------
 f
(1 row)