Oracle兼容-语法-Oracle ORDER BY兼容

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在GreatSQL中，切换到ORACLE模式后，即可支持Oracle风格的排序规则。

1. 语法

ORDER BY order_expr [, order_expr...]

order_expr :
    expr [ ASC | DESC ] [ NULLS FIRST | NULLS LAST ]

2. Oracle兼容说明

GreatSQL 和 Oracle 在 ORDER BY 排序时，对于 NULL 值的排序处理方式存在差异。

GreatSQL的做法：

• 按 ASC 排序时，NULL 排序在前;
• 按 DESC 排序时，NULL 排序在后。

Oracle的做法与GreatSQL相反，即：

• 按 ASC 排序时，NULL 排序在后;
• 按 DESC 排序时，NULL 排序在前。

可以设置 SET sql_mode = ORACLE 切换到ORACLE模式，选择使用Oracle的排序模式。

在ORACLE模式下，还可以在排序时加上 NULLS FIRST 显式设置在最终结果中将 NULL 排序在最前面。 也可以加上 NULLS LAST 显式设置在最终结果中将 NULL 排序在最后面。

3. 示例

greatsql> SET sql_mode = DEFAULT;
greatsql> CREATE TABLE t1 (id INT, desc1 VARCHAR(2), desc2 VARCHAR(2));
greatsql> INSERT INTO t1 VALUES (1, NULL, 'A'), (2, 'B', 'B'), (3, 'C', 'C'), (4, 'D', 'D');

greatsql> SELECT * FROM t1;
+------+-------+-------+
| id   | desc1 | desc2 |
+------+-------+-------+
|    1 | NULL  | A     |
|    2 | B     | B     |
|    3 | C     | C     |
|    4 | D     | D     |
+------+-------+-------+

greatsql> SELECT * FROM t1 ORDER BY desc1;
+------+-------+-------+
| id   | desc1 | desc2 |
+------+-------+-------+
|    1 | NULL  | A     |
|    2 | B     | B     |
|    3 | C     | C     |
|    4 | D     | D     |
+------+-------+-------+

greatsql> SELECT * FROM t1 ORDER BY desc1 DESC;
+------+-------+-------+
| id   | desc1 | desc2 |
+------+-------+-------+
|    4 | D     | D     |
|    3 | C     | C     |
|    2 | B     | B     |
|    1 | NULL  | A     |
+------+-------+-------+

greatsql> SELECT * FROM t1 ORDER BY desc1 NULLS FIRST;
+------+-------+-------+
| id   | desc1 | desc2 |
+------+-------+-------+
|    1 | NULL  | A     |
|    2 | B     | B     |
|    3 | C     | C     |
|    4 | D     | D     |
+------+-------+-------+

greatsql> SELECT * FROM t1 ORDER BY desc1 NULLS LAST;
+--------+-------+-------+
| id | desc1 | desc2 |
+--------+-------+-------+
|      2 | B     | B     |
|      3 | C     | C     |
|      4 | D     | D     |
|      1 | NULL  | A     |
+--------+-------+-------+

greatsql> SELECT * FROM t1 ORDER BY desc1 DESC NULLS FIRST;
+------+-------+-------+
| id   | desc1 | desc2 |
+------+-------+-------+
|    1 | NULL  | A     |
|    4 | D     | D     |
|    3 | C     | C     |
|    2 | B     | B     |
+------+-------+-------+

greatsql> SELECT * FROM t1 ORDER BY desc1 DESC NULLS LAST;
+------+-------+-------+
| id   | desc1 | desc2 |
+------+-------+-------+
|    4 | D     | D     |
|    3 | C     | C     |
|    2 | B     | B     |
|    1 | NULL  | A     |
+------+-------+-------+

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