MySQL在多个表上加入+ FK

Question

As an exercise I am working on building my first ever SQL database. I'm tried to create statements for all the tables in my database. I was wondering if any of the foreign keys could be removed?

The second question I have is how to make a query for this database that uses a join on 4 or more tables.

Any answer will be appriciated!

CREATE TABLE CLUB(
cl_id       INT     PRIMARY KEY     NOT NULL,
naam        TEXT                    NOT NULL,
adres       VARCHAR(200)                NOT NULL,
dtm_oprichta    TEXT                    NOT NULL
);


CREATE TABLE STADION(
sta_id      INT     PRIMARY KEY     NOT NULL,
cl_id       INT     REFERENCES CLUB(cl_id),
naam        TEXT                    NOT NULL,
adres       VARCHAR(200)                NOT NULL,
capaciteit  INT                 NOT NULL,
dtm_bouw    TEXT                    NOT NULL
);

CREATE TABLE TECHNISCHDIRECTEUR(
td_id       INT     PRIMARY KEY     NOT NULL,
cl_id       INT     REFERENCES CLUB(cl_id),
naam        TEXT                    NOT NULL,
adres       VARCHAR(200)                NOT NULL,
salaris     REAL                    NOT NULL,
nationaliteit   TEXT                    NOT NULL,
geslacht    CHAR                    NOT NULL,
dtm_geboorte    TEXT                    NOT NULL
);

CREATE TABLE FINANCIEELDIRECTEUR(
fd_id       INT     PRIMARY KEY     NOT NULL,
cl_id       INT     REFERENCES CLUB(cl_id),
naam        TEXT                    NOT NULL,
adres       VARCHAR(200)                NOT NULL,
salaris     REAL                    NOT NULL,
nationaliteit   TEXT                    NOT NULL,
geslacht    CHAR                    NOT NULL,
dtm_geboorte    TEXT                    NOT NULL
);


CREATE TABLE HOOFDTRAINER(
ht_id       INT     PRIMARY KEY     NOT NULL,
cl_id       INT     REFERENCES CLUB(cl_id),
td_id       INT     REFERENCES TECHNISCHDIRECTEUR(td_id),
naam        TEXT                    NOT NULL,
adres       VARCHAR(200)                NOT NULL,
salaris     REAL                    NOT NULL,
nationaliteit   TEXT                    NOT NULL,
geslacht    CHAR                    NOT NULL,
dtm_geboorte    TEXT                    NOT NULL
);


CREATE TABLE ASSISTENTTRAINER(
at_id       INT     PRIMARY KEY     NOT NULL,
cl_id       INT     REFERENCES CLUB(cl_id),
ht_id       INT     REFERENCES HOOFDTRAINER(ht_id),
naam        TEXT                    NOT NULL,
adres       VARCHAR(200)                NOT NULL,
salaris     REAL                    NOT NULL,
nationaliteit   TEXT                    NOT NULL,
geslacht    CHAR                    NOT NULL,
dtm_geboorte    TEXT                    NOT NULL
);

CREATE TABLE SPELER(
sp_id       INT     PRIMARY KEY     NOT NULL,
cl_id       INT     REFERENCES CLUB(cl_id),
ht_id       INT     REFERENCES HOOFDTRAINER(ht_id),
naam        TEXT                    NOT NULL,
adres       VARCHAR(200)                NOT NULL,
salaris     REAL                    NOT NULL,
nationaliteit   TEXT                    NOT NULL,
geslacht    CHAR                    NOT NULL,
dtm_geboorte    TEXT                    NOT NULL,
positie     TEXT                    NOT NULL,
rugnummer   INT                 NOT NULL
);

Answer 1

1) Can you remove the foreign keys. Looking at your table names:

STADION: cl_id     - Identifies a club associated to a stadium. 
TECHNISCHDIRECTEUR cl_id     - Identifies a club associated to a Technical Director
FINANCIEELDIRECTEUR cl_id     - Identifies a club associated to a Financial Director

If you remove any of those foreign keys, you will lose the ability to identify which club they belong to! So, I would say, keep them.

2) So here is an example query joining all four tables. This example shows a stadium, technical director and financial director for each club if they have them! The assumption is that each club only has one stadium\\technical director\\financial director. If they have more then one you will see the club row duplicated (but as I said this is just a simple example).

Select c.cl_id as ClubId, c.naam as Club, td.naam as TechnicalDirector, fd.naam as FinancialDirector
From Club c
left join STADION s on s.cl_id = c.cl_id
left join TECHNISCHDIRECTEUR td on td.cl_id = c.cl_id
left join FINANCIEELDIRECTEUR fd on fd.cl_id = c.cl_id