Iqembu le-MySQL lesamba ngokusetshenziswa? Incazelo enemininingwane yeqembu ngokubala kusizindalwazi se-MySQL

MySQL Hlanganisa iqembu ngokusetshenziswa?MySQL databaseIncazelo enemininingwane yeqembu ngokubala

I-MySQL GROUP NGEsitatimende

Isitatimende esithi GROUP BY sihlanganisa umphumela osethiwe ngokusekelwe kukholomu eyodwa noma ngaphezulu.

Kukholomu ehlanganisiwe singasebenzisa imisebenzi engu-COUNT, SUM, AVG, njll.

IQEMBU NGE-syntax

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

Umboniso wesibonelo

Isibonelo esikulesi sahluko sisebenzisa ukwakheka kwethebula okulandelayo kanye nedatha.Ngaphambi kokuyisebenzisa, singangenisa idatha elandelayo kusizindalwazi.

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;

Ngemuva kokuthi ukungenisa kuphumelele, yenza isitatimende se-SQL esilandelayo:

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)

Okulandelayo sisebenzisa isitatimende esithi GROUP BY ukuze siqoqe ithebula ledatha ngamagama futhi sibale ukuthi mangaki amarekhodi umuntu ngamunye anawo:

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)

Sebenzisa NGE-ROLLUP

NGE-ROLLUP ingasebenzisa izibalo ezifanayo (SUM, AVG, COUNT...) ngesisekelo sezibalo eziqoqwe.

Isibonelo, siqoqa ithebula ledatha elingenhla ngamagama, bese sibala izikhathi umuntu ngamunye angene ngazo ngemvume:

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)

Irekhodi elithi NULL limele inombolo yokungena kwawo wonke umuntu.

Singasebenzisa i-coalesce ukusetha igama elingangena esikhundleni se-NUll, i-coalesce syntax:

select coalesce(a,b,c);

Incazelo yepharamitha: uma u-a==null, khetha okuthi b; uma b==null, khetha u-c; uma a!=null, khetha u-a; uma i-abc iyize, buyisela i-null (engenanjongo).

Esibonelweni esilandelayo uma igama lingenalutho sisebenzisa isamba esikhundleni salokho:

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)