Isalathiso senqaku
MySQL Isetyenziswa njani isum group nge?MySQL databaseInkcazo eneenkcukacha yeqela ngokubala phakathi
MySQL GROUP NGENXA YESITETIMENTI
I GROUP BY ingxelo yamaqela isiphumo esisekwe kuluhlu olunye okanye ngaphezulu.
Singasebenzisa imisebenzi efana ne-COUNT, i-SUM, i-AVG, njl. kwimiqolo edibeneyo.
IQELA NGESintaksi
SELECT column_name, function(column_name) FROM table_name WHERE column_name operator value GROUP BY column_name;
Umzekelo womboniso
Imizekelo ekwesi sahluko isebenzisa olu luhlu lulandelayo lwesakhiwo kunye nedatha Ngaphambi kokusetyenziswa, sinokungenisa le datha ilandelayo kwisiseko sedatha.
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;
Emva kokuba ukungenisa elizweni kuphumelele, yenza le ngxelo ilandelayo yeSQL:
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 i-GROUP BY statement ukwenza iqela ledatha yedatha ngegama kwaye sibale ukuba zingaphi iirekhodi umntu ngamnye anazo:
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 inokwenza izibalo ezifanayo (i-SUM, i-AVG, i-COUNT...) ngokusekelwe kwidatha yamanani ehlanganisiweyo.
Umzekelo, sidibanisa le theyibhile yedatha ingentla ngegama, kwaye emva koko sibale inani lamaxesha umntu ngamnye angena:
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 NULL libonisa inani lokungena kumntu wonke.
Sinokusebenzisa i-coalesce ukuseta igama elinokuthi endaweni ye-NUll.
select coalesce(a,b,c);
Inkcazo yepharamitha: Ukuba a==null, khetha b, ukuba b==null, khetha u-c ukuba a!=null, khetha u-a ukuba u-abc azikho zombini, buyisela akukho (akukho ntsingiselo).
Kulo mzekelo ulandelayo ukuba igama lingenanto sisebenzisa itotali endaweni yoko:
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)
Ndiyathemba Chen Weiliang Blog ( https://www.chenweiliang.com/ ) kwabelwane "Iqela le-MySQL lesambuku ngokusetyenziswa?" Inkcazo eneenkcukacha yeqela ngokubala kwi-database ye-MySQL" iya kuba luncedo kuwe.
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