简单表达式

逻辑表达式

逻辑表达式的操作符和运算规则，请参见逻辑操作符。

比较表达式

常用的比较操作符，请参见比较操作符。

除比较操作符外，还可以使用以下句式结构：

• BETWEEN操作符

  a BETWEEN x AND y等效于a >= x AND a <= y

  a NOT BETWEEN x AND y等效于a < x OR a > y

• 检查一个值是不是null，可使用：

  expression IS NULL

  expression IS NOT NULL

  或者与之等价的句式结构，但不是标准的：

  expression ISNULL

  expression NOTNULL

  

  不要写expression=NULL或expression<>(!=)NULL，因为NULL代表一个未知的值，不能通过该表达式判断两个未知值是否相等。

伪列

ROWNUM

ROWNUM是一个伪列，它返回一个数字，表示从查询中获取结果的行编号。第一行的ROWNUM为1，第二行的为2，以此类推，ROWNUM的返回类型为BIGINT。ROWNUM可以用于限制查询返回的总行数，例如下面语句限制查询从Students表中返回最多10条记录。

openGauss=# CREATE TABLE Students (name varchar(20), id int) with (STORAGE_TYPE = USTORE);
openGauss=# INSERT INTO Students VALUES ('Jack', 35);
openGauss=# INSERT INTO Students VALUES ('Leon', 15);
openGauss=# INSERT INTO Students VALUES ('James', 24);
openGauss=# INSERT INTO Students VALUES ('Taker', 81);
openGauss=# INSERT INTO Students VALUES ('Mary', 25);
openGauss=# INSERT INTO Students VALUES ('Rose', 64);
openGauss=# INSERT INTO Students VALUES ('Perl', 18);
openGauss=# INSERT INTO Students VALUES ('Under', 57);
openGauss=# INSERT INTO Students VALUES ('Angel', 101);
openGauss=# INSERT INTO Students VALUES ('Frank', 20);
openGauss=# INSERT INTO Students VALUES ('Charlie', 40);

-- 输出表Students前10行数据。
openGauss=# SELECT * FROM Students WHERE rownum <= 10; 
 name  | id  
-------+-----
 Jack  |  35
 Leon  |  15
 James |  24
 Taker |  81
 Mary  |  25
 Rose  |  64
 Perl  |  18
 Under |  57
 Angel | 101
 Frank |  20
(10 rows)

openGauss=# DROP TABLE Students;
DROP TABLE

示例

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

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

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

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

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

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

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

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

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

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