Só falta o GROUP BY
SUA CONSULTA COM O GRUPO POR
SELECT
student.StudentID,
student.`Name`,
COUNT(attendance.AttendanceID) AS Total
FROM
student
LEFT JOIN attendance ON student.StudentID = attendance.StudentID
GROUP BY student.StudentID,student.`Name`;
DADOS DE AMOSTRA
DROP DATABASE IF EXISTS alishaikh; CREATE DATABASE alishaikh;
USE alishaikh
CREATE TABLE student
(
StudentID CHAR(8) NOT NULL,
Name VARCHAR(40),
PRIMARY KEY (StudentID)
);
INSERT INTO student (StudentID,Name) VALUES
('k1052280','Ali Shaikh'),('k1052287','McKenzie Roth'),
('k1052288','Dacey Sullivan'),('k1052294','Zelda Cantu'),
('k1052295','Kimberly Melton'),('k1052296','Tatianna Cantrell'),
('k1052297','Morgan Thornton'),('k1052298','Allistair Barlow'),
('k1052299','Troy Fulton');
CREATE TABLE attendance
(
AttendanceID INT NOT NULL AUTO_INCREMENT,
StudentID CHAR(8) NOT NULL,
PRIMARY KEY (AttendanceID),
KEY (StudentID)
);
INSERT INTO attendance (StudentID) VALUES
('k1052280'),('k1052280'),('k1052287'),('k1052287'),
('k1052288'),('k1052295'),('k1052295'),('k1052295');
DADOS DE AMOSTRA CARREGADOS
mysql> DROP DATABASE IF EXISTS alishaikh; CREATE DATABASE alishaikh;
Query OK, 2 rows affected (0.01 sec)
Query OK, 1 row affected (0.00 sec)
mysql> USE alishaikh
Database changed
mysql> CREATE TABLE student
-> (
-> StudentID CHAR(8) NOT NULL,
-> Name VARCHAR(40),
-> PRIMARY KEY (StudentID)
-> );
Query OK, 0 rows affected (0.03 sec)
mysql> INSERT INTO student (StudentID,Name) VALUES
-> ('k1052280','Ali Shaikh'),('k1052287','McKenzie Roth'),
-> ('k1052288','Dacey Sullivan'),('k1052294','Zelda Cantu'),
-> ('k1052295','Kimberly Melton'),('k1052296','Tatianna Cantrell'),
-> ('k1052297','Morgan Thornton'),('k1052298','Allistair Barlow'),
-> ('k1052299','Troy Fulton');
Query OK, 9 rows affected (0.00 sec)
Records: 9 Duplicates: 0 Warnings: 0
mysql> CREATE TABLE attendance
-> (
-> AttendanceID INT NOT NULL AUTO_INCREMENT,
-> StudentID CHAR(8) NOT NULL,
-> PRIMARY KEY (AttendanceID),
-> KEY (StudentID)
-> );
Query OK, 0 rows affected (0.02 sec)
mysql> INSERT INTO attendance (StudentID) VALUES
-> ('k1052280'),('k1052280'),('k1052287'),('k1052287'),
-> ('k1052288'),('k1052295'),('k1052295'),('k1052295');
Query OK, 8 rows affected (0.00 sec)
Records: 8 Duplicates: 0 Warnings: 0
SUA CONSULTA COM O GRUPO POR EXECUTADO
mysql> SELECT
-> student.StudentID,
-> student.`Name`,
-> COUNT(attendance.AttendanceID) AS Total
-> FROM
-> student
-> LEFT JOIN attendance ON student.StudentID = attendance.StudentID
-> GROUP BY student.StudentID,student.`Name`;
+-----------+-------------------+-------+
| StudentID | Name | Total |
+-----------+-------------------+-------+
| k1052280 | Ali Shaikh | 2 |
| k1052287 | McKenzie Roth | 2 |
| k1052288 | Dacey Sullivan | 1 |
| k1052294 | Zelda Cantu | 0 |
| k1052295 | Kimberly Melton | 3 |
| k1052296 | Tatianna Cantrell | 0 |
| k1052297 | Morgan Thornton | 0 |
| k1052298 | Allistair Barlow | 0 |
| k1052299 | Troy Fulton | 0 |
+-----------+-------------------+-------+
9 rows in set (0.00 sec)
mysql>
DE UMA CHANCE !!!
order by Total
após o grupo porGROUP BY
define o padrão paraORDER BY
. Às vezes não. Então,ORDER BY
também pode ser usado !!!TestResult
com colunas de(StudentID, Result)
. Desejo também participarTestResult
e, eventualmente, obterCOUNT(TestResult.Result)
para cada aluno. Por algum motivo, estou obtendo valores em excesso para a COUNT, enquanto as subconsultas retornaram as COUNTs corretas.DISTINCT
palavras - chave. Qual seria o motivo dos COUNTs retornarem algo diferente (geralmente mais alto e, é claro, errado) do que o padrão de subconsulta?