Grupp tas-somma MySQL skont l-użu? Spjegazzjoni dettaljata tal-grupp skond l-għadd fid-database MySQL

MySQL Grupp ta' somma skond l-użu?database MySQLSpjegazzjoni dettaljata tal-grupp skond l-għadd

Dikjarazzjoni MySQL GROUP BY

L-istqarrija GROUP BY tiġbor is-sett tar-riżultat ibbażat fuq kolonna waħda jew aktar.

Fuq il-kolonna raggruppata nistgħu nużaw funzjonijiet COUNT, SUM, AVG, eċċ.

GRUPP MINN sintassi

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

Eżempju ta' dimostrazzjoni

L-eżempju f'dan il-kapitolu juża l-istruttura u d-dejta tat-tabella li ġejja Qabel ma nużawha, nistgħu nimportaw id-dejta li ġejja fid-database.

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;

Wara li l-importazzjoni tirnexxi, wettaq id-dikjarazzjoni SQL li ġejja:

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)

Imbagħad nużaw id-dikjarazzjoni GROUP BY biex niġbru t-tabella tad-dejta bl-isem u ngħoddu kemm għandha rekords kull persuna:

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)

Uża MA ROLLUP

WITH ROLLUP tista' timplimenta l-istess statistika (SUM, AVG, COUNT...) fuq il-bażi ta' statistika raggruppata.

Pereżempju, niġbru t-tabella tad-dejta ta’ hawn fuq bl-isem, u mbagħad ngħoddu n-numru ta’ drabi li kull persuna illoggjat:

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)

Ir-rekord NULL jirrappreżenta n-numru ta' logins għal kulħadd.

Nistgħu nużaw coalesce biex nissettjaw isem li jista' jissostitwixxi NUll, coalesce sintassi:

select coalesce(a,b,c);

Deskrizzjoni tal-parametru: jekk a==null, agħżel b; jekk b==null, agħżel c; jekk a!=null, agħżel a; jekk abc huwa null, irritorna null (bla sens).

Fl-eżempju li ġej jekk l-isem ikun vojt, nużaw it-total minflok:

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)