感謝分享：http://blog.itpub.net/13379967/viewspace-715701/
在統計需求中 有時往往需要對區間進行分組
mysql中可以利用elt函數來實現此類需求
mysql> select * from k1;
+------+------+
| id | yb |
+------+------+
| 1 | 100 |
| 2 | 11 |
| 3 | 5 |
| 4 | 501 |
| 5 | 1501 |
| 6 | 1 |
+------+------+
現在要進行統計，小於100的，100~500的，500~1000的，1000以上的，這各個區間的id數
利用interval划出4個區間
再利用elt函數將4個區間分別返回一個列名
mysql> select elt(interval(d.yb,0, 100, 500, 1000), 'less100', '100to500', '500to1000', 'more1000') as yb_level, count(d.id) as cnt
-> from k1 d
-> group by elt(interval(d.yb, 0, 100, 500, 1000), 'less100', '100to500', '500to1000', 'more1000K');
+-----------+-----+
| yb_level | cnt |
+-----------+-----+
| 100to500 | 1 |
| 500to1000 | 1 |
| less100 | 3 |
| more1000 | 1 |
+-----------+-----+
4 rows in set (0.00 sec)
如果需要按從小到大排序的話 可以在列名定義時稍加一個首字符 對各檔區間進行排序
mysql> select elt(interval(d.yb,0, 100, 500, 1000), '1/less100', '2/100to500', '3/500to1000', '4/more1000') as yb_level, count(d.id) as cnt
-> from k1 d
-> group by elt(interval(d.yb, 0, 100, 500, 1000), '1/less100', '2/100to500', '3/500to1000', '4/more1000K');
+-------------+-----+
| yb_level | cnt |
+-------------+-----+
| 1/less100 | 3 |
| 2/100to500 | 1 |
| 3/500to1000 | 1 |
| 4/more1000 | 1 |
+-------------+-----+
4 rows in set (0.00 sec)
附elt函數格式：
ELT(N,str1,str2,str3,...)
如果N= 1，返回str1，如果N= 2，返回str2，等等。如果N小於1或大於參數個數，返回NULL。ELT()是FIELD()反運算。
mysql> select ELT(1, 'ej', 'Heja', 'hej', 'foo');
-> 'ej'
mysql> select ELT(4, 'ej', 'Heja', 'hej', 'foo');
-> 'foo'
interval函數格式：
INTERVAL()
Return the index of the argument that is less than the first argument（小於后面的某個參數，就返回這個參數的前一個位置數字）
INTERVAL(N,N1,N2,N3,...)
Returns 0 if N < N1, 1 if N < N2 and so on or -1 if N is NULL. All arguments are treated as integers. It is required that N1 < N2 < N3 < ... < Nn for this function to work correctly. This is because a binary search is used (very fast).
mysql> SELECT INTERVAL(23, 1, 15, 17, 30, 44, 200); （23小於30，30的位置是4，於是返回3）
-> 3
mysql> SELECT INTERVAL(10, 1, 10, 100, 1000);
-> 2
mysql> SELECT INTERVAL(22, 23, 30, 44, 200);
-> 0