mysql将1张表加入另外2张表

Question

I need to compare actual hours to estimated hours for each task in a project. The information lives in 3 different tables

Tasks Table: Contains all the project task information

• Task ID (key)
• Task Name
• Project Name

Timesheets Table: Contains the actual hours logged to the task

• Task ID
• Task Name
• Hours (sum) [multiple Timesheets records for each task]

Task Owner Table: Contains the estimated hours assigned to the task

• Task Id
• Task Name
• Estimated Hour (sum) [multiple Task Owner records for each task]

I need to pull the task information from the Tasks table, the sum of actual hours from the timesheets table, and sum of estimated hours from the Task Owner table

Here is what it should look like:

Project Name | Task Name | Actual Hours | Estimated Hours

Here is my starting query which shows me the total actual hours per project task. Now I need to know how to add a column for total estimated hours

SELECT
tasks."Project Name" AS "Project Name",
tasks."Task Name" AS "Task Name",
sum("Timesheets"."Hours") AS "Hours"
FROM "Tasks" join "Timesheets" ON "Tasks"."Task ID" = "Timesheets"."Task ID"
GROUP BY tasks."Project Name", tasks."Task Name"

Answer 1

it would be possible to assign the aggregate columns to select subqueries:

SELECT
tasks."Project Name" AS "Project Name",
tasks."Task Name" AS "Task Name",
(select sum("Timesheets"."Hours" FROM "Timesheets" where "Timesheets"."Task ID" = "Tasks"."Task ID") as "Hours",
(select sum("Task Owner"."Hours" FROM "Timesheets" where "Task Owner"."Task ID" = "Tasks"."Task ID") as "Estimated Hours"
FROM "Tasks"

Answer 2

You should add another join:

SELECT
    tasks.`Project Name` AS `Project Name`,
    tasks.`Task Name` AS `Task Name`,
    sum(`Timesheets`.`Hours`) AS `Hours`
    sum(`Task Owner`.Estimated Hour`) AS `Estimated Hour`
    FROM `Tasks` 
    join `Timesheets` ON `Tasks`.`Task ID` = `Timesheets`.`Task ID`
    join `Task Owner`  ON `Tasks`.`Task ID` = `Task Owner`.`Task ID`

GROUP BY tasks.`Project Name`, tasks.`Task Name`

And I would like to suggest you use of backtics instead of double quote for table and column names.

Answer 3

It may help here to pre-aggregate your actual and estimated hours in a subquery and THEN join it in. This is especially important if you have many rows for both your Task Owner and Timesheets table for each distinct task id .

SELECT
    tasks.`Project Name` AS `Project Name`,
    tasks.`Task Name` AS `Task Name`,
    actual.hours,
    estimated.hours,
FROM Tasks 
    INNER JOIN (SELECT `TASK ID`, SUM(Hours) as hours FROM Timesheets GROUP BY `TASK ID`) actual
        ON Tasks.`Task ID` = actual.`Task ID`
    INNER JOIN (SELECT `TASK ID`, SUM(`Estimated Hour`) as hours FROM `Task Owner` GROUP BY `TASK ID`) estimated 
        ON Tasks.`TASK ID` = estimated.`TASK ID`

Going with this method insures that each subquery returns a unique record for each task id with the sum of estimated or actual hours.

Answer 4

You can't just join the three tables directly together since both "Timesheets" and "Task Owner" can have duplicate values in column "task id". To avoid generating more rows when joining, you need to group those tables beforehand. That could be done with subqueries, but using WITH results in clean code.

WITH actual AS (
    SELECT
        "Task ID",
        SUM(Hours) AS hours
    FROM "Task Owner"
    GROUP BY "Task ID"
),

estimated AS (
    SELECT
        "Task ID",
        SUM(Hours) AS hours
    FROM Timesheets
    GROUP BY "Task ID"
)

SELECT 
    t."project name"      AS "Project Name", 
    t."task name"         AS "Task Name", 
    a.hours               AS "Actual Hours"
    e.hours               AS "Estimated Hours"
FROM tasks t
LEFT JOIN actual a
    ON t."task id" = a."task id" 
LEFT JOIN estimated e
    ON t."task id" = e."task id" 
GROUP  BY tasks."project name", 
    tasks."task name"