§ Oracle兼容-语法-Oracle ORDER BY兼容
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 ]
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§ 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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