MySQL nchikota otu site na ojiji? Nkọwa zuru ezu nke otu site na ọnụ na nchekwa data MySQL

Kedu otu esi eji SUM GROUP BY na MySQL ? Nkọwa zuru ezu nke GROUP BY COUNT na nchekwa data MySQL.

MySQL GROUP BY nkwupụta

GROUP BY nkwupụta na-achịkọta nsonaazụ atọrọ dabere na otu kọlụm ma ọ bụ karịa.

Na kọlụm agbakọtara anyị nwere ike iji ọrụ COUNT, SUM, AVG, wdg.

GROUP BY syntax

SELECT column_name, function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name;

Ngosipụta ihe atụ

Ihe atụ dị n'isiakwụkwọ a na-eji nhazi tebụl na data ndị a. Tupu iji ya, anyị nwere ike ibubata data ndị a na nchekwa data.

SET NAMES utf8;
SET FOREIGN_KEY_CHECKS = 0;
-- ----------------------------
--  Table structure for `employee_tbl`
-- ----------------------------
DROP TABLE IF EXISTS `employee_tbl`;
CREATE TABLE `employee_tbl` (
  `id` int(11) NOT NULL,
  `name` char(10) NOT NULL DEFAULT '',
  `date` datetime NOT NULL,
  `singin` tinyint(4) NOT NULL DEFAULT '0' COMMENT '登录次数',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
-- ----------------------------
--  Records of `employee_tbl`
-- ----------------------------
BEGIN;
INSERT INTO `employee_tbl` VALUES ('1', '小明', '2016-04-22 15:25:33', '1'), ('2', '小王', '2016-04-20 15:25:47', '3'), ('3', '小丽', '2016-04-19 15:26:02', '2'), ('4', '小王', '2016-04-07 15:26:14', '4'), ('5', '小明', '2016-04-11 15:26:40', '4'), ('6', '小明', '2016-04-04 15:26:54', '2');
COMMIT;

SET FOREIGN_KEY_CHECKS = 1;

Mgbe mbubata ahụ mechara nke ọma, mebie nkwupụta SQL ndị a:

mysql> set names utf8;
mysql> SELECT * FROM employee_tbl;
+----+--------+---------------------+--------+
| id | name   | date                | singin |
+----+--------+---------------------+--------+
|  1 | 小明 | 2016-04-22 15:25:33 |      1 |
|  2 | 小王 | 2016-04-20 15:25:47 |      3 |
|  3 | 小丽 | 2016-04-19 15:26:02 |      2 |
|  4 | 小王 | 2016-04-07 15:26:14 |      4 |
|  5 | 小明 | 2016-04-11 15:26:40 |      4 |
|  6 | 小明 | 2016-04-04 15:26:54 |      2 |
+----+--------+---------------------+--------+
6 rows in set (0.00 sec)

Anyị na-eji nkwupụta GROUP BY chịkọta tebụl data n'aha wee gụọ ndekọ ole onye ọ bụla nwere:

mysql> SELECT name, COUNT(*) FROM   employee_tbl GROUP BY name;
+--------+----------+
| name   | COUNT(*) |
+--------+----------+
| 小丽 |        1 |
| 小明 |        3 |
| 小王 |        2 |
+--------+----------+
3 rows in set (0.01 sec)

Jiri ya na mpịakọta

NA ROLLUP nwere ike mejuputa otu ọnụ ọgụgụ ahụ (SUM, AVG, COUNT...) dabere na ọnụ ọgụgụ agbakọtara.

Dịka ọmụmaatụ, anyị na-ejikọta tebụl data dị n'elu n'aha, wee gụọ ugboro ole onye ọ bụla banyere:

mysql> SELECT name, SUM(singin) as singin_count FROM  employee_tbl GROUP BY name WITH ROLLUP;
+--------+--------------+
| name   | singin_count |
+--------+--------------+
| 小丽 |            2 |
| 小明 |            7 |
| 小王 |            7 |
| NULL   |           16 |
+--------+--------------+
4 rows in set (0.00 sec)

Ndekọ NULL na-anọchi anya ọnụọgụ nbanye maka onye ọ bụla.

Anyị nwere ike iji coalesce ịtọ aha nwere ike dochie NUll, syntax coalesce:

select coalesce(a,b,c);

Nkọwa paramita: ọ bụrụ == efu, họrọ b; ọ bụrụ b== efu, họrọ c; ọ bụrụ a! = efu, họrọ a; ọ bụrụ na abc efu, laghachi efu (enweghị isi).

N'ihe atụ na-esonụ ọ bụrụ na aha efu, anyị na-eji mkpokọta kama:

mysql> SELECT coalesce(name, '总数'), SUM(singin) as singin_count FROM  employee_tbl GROUP BY name WITH ROLLUP;
+--------------------------+--------------+
| coalesce(name, '总数') | singin_count |
+--------------------------+--------------+
| 小丽                   |            2 |
| 小明                   |            7 |
| 小王                   |            7 |
| 总数                   |           16 |
+--------------------------+--------------+
4 rows in set (0.01 sec)