Sehlopha sa kakaretso sa MySQL ka tšebeliso? Tlhaloso e qaqileng ea sehlopha ka palo ho database ea MySQL

MySQL Mokhoa oa ho sebelisa sehlopha sa kakaretso joang?MySQL databaseTlhaloso e qaqileng ea sehlopha ka count in

MySQL GROUP BY statement

Polelo ea GROUP BY e kopanya sephetho se ipapisitseng le kholomo e le 'ngoe kapa ho feta.

Re ka sebelisa mesebetsi e kang COUNT, SUM, AVG, joalo-joalo likholomong tse hlophisitsoeng.

SEHLOPHA KA syntax

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

Pontšo ea mohlala

Mehlala e khaolong ena e sebelisa sebopeho sa tafole le lintlha tse latelang Pele re li sebelisa, re ka kenya lintlha tse latelang polokelong ea litaba.

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;

Ka mor'a hore ho kenya thepa e atlehe, phethahatsa polelo e latelang ea SQL:

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)

Ka mor'a moo re sebelisa polelo ea GROUP BY ho hlophisa tafole ea data ka mabitso le ho bala hore na motho ka mong o na le lirekoto tse kae:

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)

Sebelisa KA ROLLUP

KA ROLLUP e ka etsa lipalo-palo tse tšoanang (SUM, AVG, COUNT...) ho ipapisitsoe le lintlha tsa lipalo tse hlophisitsoeng.

Mohlala, re hlophisa tafole ea data e kaholimo ka mabitso, ebe re bala makhetlo ao motho ka mong a kenang ho ona:

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)

Rekoto ea NULL e bonts'a palo ea li-logins bakeng sa motho e mong le e mong.

Re ka sebelisa coalesce ho theha lebitso le ka nkang sebaka sa syntax ea NUll.

select coalesce(a,b,c);

Tlhaloso ea paramethara: Haeba a==null, khetha b; haeba b==null, khetha c; haeba a!=null, khetha a; haeba b c e le lefeela, khutlela null (e se nang moelelo).

Mohlaleng o latelang haeba lebitso le se na letho re sebelisa kakaretso ho e-na le hoo:

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)