Buidheann suim MySQL a rèir cleachdadh? Mìneachadh mionaideach air buidheann a rèir cunntais ann an stòr-dàta MySQL

MySQL Suim buidheann a rèir cleachdadh?Stòr-dàta MySQLMìneachadh mionaideach air buidheann a rèir cunntais

Buidheann MySQL A rèir aithris

Bidh an aithris GROUP BY a’ cruinneachadh an t-seata thoraidhean stèidhichte air aon cholbh no barrachd.

Air a’ cholbh cruinnichte is urrainn dhuinn gnìomhan COUNT, SUM, AVG, msaa a chleachdadh.

GRÙP A THAOBH syntax

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

Eisimpleir taisbeanadh

Tha an eisimpleir sa chaibideil seo a' cleachdadh structar a' chlàir agus an dàta a leanas: Mus cleachd sinn e, is urrainn dhuinn an dàta a leanas a thoirt a-steach dhan stòr-dàta.

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;

Às deidh an in-mhalairt a bhith soirbheachail, cuir an gnìomh an aithris SQL a leanas:

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)

An uairsin bidh sinn a’ cleachdadh an aithris GROUP BY gus an clàr dàta a chuir còmhla a rèir ainm agus cunntadh cia mheud clàr a th’ aig gach neach:

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)

Cleachd LE ROLLUP

Faodaidh LE ROLLUP na h-aon staitistigean a chuir an gnìomh (SUM, AVG, COUNT ...) air bunait staitistig buidhne.

Mar eisimpleir, bidh sinn a’ cruinneachadh a’ chlàr dàta gu h-àrd a rèir ainm, agus an uairsin a’ cunntadh an àireamh de thursan a tha gach neach air logadh a-steach:

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)

Tha an clàr NULL a’ riochdachadh na h-àireimh de logadh a-steach airson a h-uile duine.

Is urrainn dhuinn coalesce a chleachdadh gus ainm a shuidheachadh a dh’ fhaodadh a dhol an àite NUll, co-chòrdadh coalesce:

select coalesce(a,b,c);

Tuairisgeul paramadair: ma tha a == null, tagh b; ma tha b = = null, tagh c; ma tha a! = null, tagh a; ma tha abc null, till null (gun chiall).

Anns an eisimpleir a leanas ma tha an t-ainm falamh cleachdaidh sinn an àireamh iomlan na àite:

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)