MySQL中的查询

Question

我想获取所有时薪为17.5的员工，项目名称为HR System。

employee table: employee_id (PK), employee_name
project table: project_code (PK), project_title
project_team: project_code (FK), employee_id (FK), hourly_rate

我尝试使用以下查询获取语法错误。 我在哪里做错了？

SELECT employee.`department_id`, employee.`employee_name`, project_team.`hourly_rate`, project.`project_title`
FROM
(employee INNER JOIN project_team ON employee.`employee_id` = project_team.`employee_id`)
AND
(project INNER JOIN project_team ON project.`project_code` = project_team.`project_code`)
AND
(project_team.`hourly_rate` = 17.50)
AND
(project.`project_title` = "HR System");

Answer 1

您正在FROM语句中使用AND子句。

SELECT
    employee.`department_id`,
    employee.`employee_name`,
    project_team.`hourly_rate`,
    project.`project_title`
FROM project_team
INNER JOIN employee
    ON employee.`employee_id` = project_team.`employee_id`
INNER JOIN project
    ON project.`project_code` = project_team.`project_code`
WHERE project_team.`hourly_rate` = 17.50
    AND project.`project_title` = "HR System";

Answer 2

您不需要将AND放在JOINS附近。

尝试这个：

SELECT
  employee.`department_id`,
  employee.`employee_name`,
  project_team.`hourly_rate`,
  project.`project_title`
FROM project_team
INNER JOIN employee
  ON employee.`employee_id` = project_team.`employee_id`
INNER JOIN project
 ON project.`project_code` = project_team.`project_code`
WHERE project_team.`hourly_rate` = 17.50
AND project.`project_title` = "HR System";;

Answer 3

使用此语句

SELECT employee.`department_id`, employee.`employee_name`, project_team.`hourly_rate`, project.`project_title`
FROM employee 
INNER JOIN project_team ON employee.`employee_id` = project_team.`employee_id`
inner join project ON project.`project_code` = project_team.`project_code`
where
(project_team.`hourly_rate` = 17.50)
AND
(project.`project_title` = "HR System");

您不能在from子句中使用AND 。 您必须使用正确的join语法。

Answer 4

我认为这个查询可以完成工作：

SELECT employee.`employee_id`, employee.`employee_name`, project_team.`hourly_rate`, project.`project_title`
FROM employee
LEFT JOIN project_team
ON employee.`employee_id` = project_team.`employee_id`
LEFT JOIN project
ON project.`project_code` = project_team.`project_code`
WHERE project_team.`hourly_rate` = 17.50
AND project.`project_title` = "HR System";

当您要LEFT JOIN 2个表时，请使用LEFT JOIN ，并且当您要测试值时不要忘记WHERE 。