使用具有一对多关系的连接最小化SQL查询

Question

So let me preface this by saying that I'm not an SQL wizard by any means. What I want to do is simple as a concept, but has presented me with a small challenge when trying to minimize the amount of database queries I'm performing.

Let's say I have a table of departments. Within each department is a list of employees.

What is the most efficient way of listing all the departments and which employees are in each department.

So for example if I have a department table with:

id   name
1    sales
2    marketing

And a people table with:

id   department_id   name
1    1               Tom
2    1               Bill
3    2               Jessica
4    1               Rachel
5    2               John

What is the best way list all departments and all employees for each department like so:

Sales

• Tom
• Bill
• Rachel

Marketing

• Jessica
• John

Pretend both tables are actually massive. (I want to avoid getting a list of departments, and then looping through the result and doing an individual query for each department). Think similarly of selecting the statuses/comments in a Facebook-like system, when statuses and comments are stored in separate tables.

Answer 1

You can get it all in a single query with a simple join, eg:

SELECT   d.name AS 'department', p.name AS 'name'
FROM     department d
  LEFT JOIN people p ON p.department_id = d.id
ORDER BY department

This returns all the data, but it's a bit of a pain to consume, since you'll have to iterate through every person anyway. You can go further and group them together:

SELECT   d.name AS 'department',
         GROUP_CONCAT(p.name SEPARATOR ', ') AS 'name'
FROM     department d
  LEFT JOIN people p ON p.department_id = d.id
GROUP BY department

You'll get something like this as the output:

department | name
-----------|----------------
sales      | Tom, Bill, Rachel
marketing  | Jessica, John

Answer 2

SELECT d.name, p.name
FROM department d
JOIN people p ON p.department_id = d.id

I suggest also reading a SQL Join tutorial or three. This is a very common and very basic SQL concept that you should understand thoroughly.

Answer 3

This is normally done in a single query:

SELECT DepartmentTable.Name, People.Name from DepartmentTable 
INNER JOIN People
ON DepartmentTable.id = People.department_id
ORDER BY DepartmentTable.Name

This will suppress empty departments. If you want to show empty departments, change INNER to LEFT OUTER