Mysql – How to select two columns from two different tables using foreign keys from another third table

join;MySQLselect

I am using MYSQL and here is the schema that I have:

Here is my first table user

+-----+------------------+
| id  | name             |
+-----+------------------+
| 101 | Daniel Radcliffe |
| 102 | Elijah Wood      |
| 103 | Robert Downey Jr |
+-----+------------------+

Here is my second table movie

+----+---------------------+
| id | name                |
+----+---------------------+
|  1 | Harry Potter 1      |
|  2 | Harry Potter 2      |
|  3 | Harry Potter 3      |
|  4 | Lord of the Rings 1 |
|  5 | Lord of the Rings 2 |
|  6 | Lord of the Rings 3 |
|  7 | Iron Man 1          |
|  8 | Iron Man 2          |
+----+---------------------+

Here is my last table list

+---------+----------+ 
| user_id | movie_id | 
+---------+----------+ 
|     101 |        1 | 
|     101 |        2 | 
|     101 |        3 | 
|     102 |        4 | 
|     102 |        5 | 
|     102 |        6 | 
|     103 |        7 | 
|     103 |        8 | 
+---------+----------+

Now I want to display name and movie side by side something like this:

+------------------+--------------------+
| Daniel Radcliffe | Harry Potter 1     |
| Daniel Radcliffe | Harry Potter 2     |
| Daniel Radcliffe | Harry Potter 3     |
| Elijah Wood      | Lord of the Rings 1|
| Elijah Wood      | Lord of the Rings 2|
| Elijah Wood      | Lord of the Rings 3|
| Robert Downey Jr | Iron Man 1         |
| Robert Downey Jr | Iron Man 2         |
+------------------+--------------------+

The list table has foreign keys for user_id and movie_id which reference user.id and movie.id respectively. So how can I use JOINs to display the above data.

Best Answer

SELECT user.name, movie.name
FROM user, movie, list
WHERE list.user_id  = user.id
  AND list.movie_id = movie.id
ORDER BY 1,2;

Or, if you need to see the word JOIN in the query text

SELECT user.name, movie.name
FROM user
JOIN list  ON list.user_id  = user.id
JOIN movie ON list.movie_id = movie.id
ORDER BY 1,2;

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Mysql – Database Design: What is the best structure for storing site offices and locations

Here is some vanilla SQL:

CREATE TABLE RoomTypes
(
 RoomType VARCHAR(12) NOT NULL,
 UNIQUE (RoomType)
);

CREATE TABLE Zones
(
 Zone VARCHAR(10) NOT NULL,
 UNIQUE (Zone)
);

CREATE TABLE Rooms
(
 RoomType VARCHAR(12) NOT NULL,
 Zone VARCHAR(10) NOT NULL, 
 Name VARCHAR(30) NOT NULL, 
 UNIQUE (RoomType, Zone, Name), 
 FOREIGN KEY (RoomType) REFERENCES RoomTypes (RoomType),
 FOREIGN KEY (Zone) REFERENCES Zones (Zone)
);

CREATE TABLE NumberedRooms 
(
 RoomType VARCHAR(12) NOT NULL,
 Zone VARCHAR(10) NOT NULL, 
 Name VARCHAR(30) NOT NULL, 
 CHECK (RoomType = 'Numbered'),
 UNIQUE (Name), 
 UNIQUE (RoomType, Zone, Name),
 FOREIGN KEY (RoomType, Zone, Name) 
    REFERENCES Rooms (RoomType, Zone, Name)
);

CREATE TABLE AncillaryRooms 
(
 RoomType VARCHAR(12) NOT NULL,
 Zone VARCHAR(10) NOT NULL, 
 Name VARCHAR(30) NOT NULL, 
 CHECK (RoomType = 'Ancillary'),
 UNIQUE (Zone, Name), 
 UNIQUE (RoomType, Zone, Name),
 FOREIGN KEY (RoomType, Zone, Name) 
    REFERENCES Rooms (RoomType, Zone, Name)
);

The CHECK constraints will not be tested by mySQL e.g. do the tests yourself using triggers. Consider adding other tests e.g. that attribute Name in table NumberedRooms represents an integer.

The idea that every row in the supertype table Rooms will have exactly one row in the union of AncillaryRooms and NumberedRooms. This is merely implied e.g. have 'helper' procs to add rows to both super- and subtype tables as a single operation and use triggers to ensure it is done.

Note NumberedRooms has a simple key Name alone, whereas AncillaryRooms has a compound key on (Zone, Name). All three (non-lookup) tables have a key on (RoomType, Zone, Name) throughout, allowing further subtype tables to referencing them and maintain integrity by further testing for valid RoomType values.

Postgresql – How to remove data from multiple tables with different foreign keys without using ON DELETE CASCADE

You're interpreting the semantics of the delete statement incorrectly. When a using clause is used, it doesn't mean that records will also be deleted from those tables. Instead, those tables are purely used to join to in order to determine which rows need to be deleted from Users.

You basically have three choices:

deleting child rows in a before delete on Users trigger.
on delete cascade constraints.
execute multiple delete statements on the various tables involved, in the right order.

My preference, in certain cases, is actually for the on delete cascade constraints, but I don't use them everywhere: just for the situation where it makes sense to be able to remove all of the children of a given parent in one go. I might use it for "invoices" and "invoice_lines".

When you take this approach you need to be sure that only users who really need to be able to delete from the parent table have that privilege -- no users or applications logging in as table owners!

Best Answer

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Mysql – Database Design: What is the best structure for storing site offices and locations

Postgresql – How to remove data from multiple tables with different foreign keys without using ON DELETE CASCADE

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