Follow a loan through its keys
three-table joins, and which branch is which
A loan names a member, a tool and a staff member by their keys, and each of those has a branch of its own.
Can you
- Start from the table the report is about and follow each foreign key to its parent
- Refuse a join on a shared column that is not a relationship
- Choose which of three branches a question is really about
The reading
Read the whole chapterFollow a loan through its keys
SELECT l.id AS loan_id, m.first_name AS member_first_name, t.name AS tool_name
FROM loans l
JOIN members m ON m.id = l.member_id
JOIN tools t ON t.id = l.tool_id
WHERE l.id <= 5
ORDER BY l.id;A loan row names a member, a tool and a staff member by their keys. When each output row should describe a loan, start from loans and follow each foreign key to its parent: JOIN members on l.member_id, JOIN tools on l.tool_id, JOIN staff on l.staff_id. Three joins, one row per loan, 120 rows. The hub is the table whose rows the report is about.
Resist the shortcut. Members and tools both carry a branch_id, so you could pair them on it. That pairs every member with every tool at the same branch, 341 rows that mean nothing: a shared branch is not a loan. For every join you write, name the fact that makes the pair real.
The Tool Library has three different branches in one loan: the branch that owns the tool, the home branch of the member, and the branch where the staff member works. They are three relationships, three joins to the branches table under three aliases. The director's replacement-value report attributes value to the branch that owns the tool. Pick the branch the question is about, not the one that is easiest to reach.
Rule of thumb. Start from the table the report is about and follow each foreign key to its parent.
The ladder
Start at the first rung even if the idea looks familiar: it is answerable from the reading alone, and the climb is where the difficulty lives. A wrong answer sends you back to the paragraph that teaches it, then to a hint, then to the worked answer.
Learn
- Practise this rung
Level 1: Watch it answered
- Practise this rung
Level 2: Fill the blank
Practise
- Practise this rung
Level 3: Predict the result
- Practise this rung
Level 4: Write it yourself
Master
- Practise this rung
Level 5: Repair it
- Practise this rung
Level 6: Transfer it
Keep practising
The whole bank for this concept, in any order, with the same review, hint and worked answer.
Practice here is evidence for you and your instructor, never a grade. Grades live in Canvas.