§ 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 ]
1
2
3
4

§ 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:

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

Oracle的做法与GreatSQL相反,即:

  • ASC 排序时,NULL 排序在后;
  • 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     |
+------+-------+-------+
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73

扫码关注微信公众号

greatsql-wx