SQL Chaining joins - mysql

Question

EDIT: The isssue was mismatching data the B-ids in tables A_B and B_C had no overlap
I have the following tables:
A(id, …)
A_B(a_id, b_id)
B_C(b_id, c_id)
C_D(c_id, d_id)
D(id, value, …)
Where I want some rows from A based on the value of the row in D.
The tables A_B, B_C and C_D are just id mappings from one table to another.
I was trying to get to something that would look something like this:
select * from A where D.value = "true"
I got this far:
select * from A
inner join A_B on A_B.a_id = A.id
inner join B_C on B_C.b_id = A_B.b_id
which is just an empty table.
I am starting to think that I am approaching this issue in the wrong way or perhaps have misunderstood how one should go about joining tables.

Answer 1

What you've got works OK if your data is coherent. Here's a version of your database and query which demonstrates that. For simplicity, I inferred the existence of tables b and c, and made the various columns in the X_Y style tables into foreign keys to the single-letter tables.
CREATE TABLE a
(
id INTEGER NOT NULL PRIMARY KEY,
info VARCHAR(20) NOT NULL
);
CREATE TABLE b
(
id INTEGER NOT NULL PRIMARY KEY,
data VARCHAR(20) NOT NULL
);
CREATE TABLE C
(
id INTEGER NOT NULL PRIMARY KEY,
extra VARCHAR(20) NOT NULL
);
CREATE TABLE d
(
id INTEGER NOT NULL PRIMARY KEY,
value VARCHAR(20) NOT NULL
);
CREATE TABLE a_b
(
a_id INTEGER NOT NULL REFERENCES a,
b_id INTEGER NOT NULL REFERENCES b,
PRIMARY KEY (a_id, b_id)
);
CREATE TABLE b_c
(
b_id INTEGER NOT NULL REFERENCES b,
c_id INTEGER NOT NULL REFERENCES C,
PRIMARY KEY(b_id, c_id)
);
CREATE TABLE c_d
(
c_id INTEGER NOT NULL REFERENCES C,
d_id INTEGER NOT NULL REFERENCES d,
PRIMARY KEY(c_id, d_id)
);
INSERT INTO a VALUES(1, "Quasimodo");
INSERT INTO b VALUES(20, "Quiet");
INSERT INTO C VALUES(333, "Contemporaneous");
INSERT INTO d VALUES(4444, "true");
INSERT INTO a_b VALUES(1, 20);
INSERT INTO b_c VALUES(20, 333);
INSERT INTO c_d VALUES(333, 4444);
SELECT *
FROM a
JOIN a_b ON a_b.a_id = a.id
JOIN b_c ON b_c.b_id = a_b.b_id
JOIN c_d ON c_d.c_id = b_c.c_id
JOIN d ON d.id = c_d.d_id
WHERE d.value = "true";
For the given data, that produces:
1 Quasimodo 1 20 20 333 333 4444 4444 true
So, if the data is correct, the query you were building can produce an answer. However, if you were getting an empty table on your incomplete query, then there is a data problem in your tables — or (outside chance) your outline schema misled us.
Testing performed on a Mac running macOS High Sierra 10.13.4, using Informix 12.10.FC6, but using what is believed to be a subset of SQL common to Informix and MySQL.