If I have a two tables like this:
CREATE TABLE users
(
id INTEGER PRIMARY KEY,
username TEXT UNIQUE NOT NULL
);
CREATE TABLE games
(
id INTEGER PRIMARY KEY,
player1 INTEGER REFERENCES users,
player2 INTEGER REFERENCES users,
comment TEXT
);
How can I insert a new row into the games table given two usernames?
The primary keys of the users would need to be looked up by their names in the users table first and then inserted into the games table. What's the best way to do this?
So, instead of
INSERT INTO games (player1, player2)
VALUES (1, 2);
how can I combine this with looking up the id values from usernames?
You could use subqueries to lookup the two usernames based on their ids:
INSERT INTO games (player1, player2, comment)
SELECT
(SELECT username FROM users WHERE id = 1),
(SELECT username FROM users WHERE id = 2),
'good luck';
I want to create two tables Category and Product in SQLite.
Category table should be parent and Product table should be a child.
I created two tables in this way, and I do not know it is a good approach:
CREATE TABLE Category (
id BIGINT PRIMARY KEY,
category_name VARCHAR,
category_description VARCHAR,
image_path VARCHAR
):
CREATE TABLE Product (
product_id integer PRIMARY KEY,
product_name VARCHAR,
product_description VARCHAR,
product_numberOfProduct integer,
product_image_path VARCHAR,
product_price integer,
category_id integer NOT NULL,
FOREIGN KEY (category_id) REFERENCES Category(id)
);
I want to create a query for searching all products from category_name and something like that.
Your approach is almost spot on. The major deficiency in your solution is that if a given product can belong to more than one category, then your schema would be duplicating all of that product's metadata for each product-category relationship. The standard way to deal with this is to create a third junction table, which mainly exists to store the relationships between products and their categories. So, your junction table might look something like this:
CREATE TABLE Product_Category (
product_id integer,
category_id integer,
PRIMARY KEY (product_id, category_id)
)
From your current Product table you would remove the reference to the category_id.
As an example query, if you wanted to find out all category names of a given product, you could try:
SELECT
c.category_name
FROM Category c
INNER JOIN Product_Category pc
ON c.id = pc.category_id
INNER JOIN Product p
ON pc.product_id = p.product_id
WHERE
p.product_name = 'some product name';
I have a database which contains many tables and these tables can be added or removedd any time.So I give each of them a different name like Table1,Table2,...
but it's uncomfortable to use these table because sometime I forget what infomation was stored in Table1
So I want something to differentiate these all tables, some property that I can be specified when I create a table and I can use to access a specific table when I need to fetch informations from that table
As one comment says, you could create a table for notes on the other tables:
CREATE TABLE notes (
id INT AUTO_INCREMENT,
table_name VARCHAR(64),
note VARCHAR(255),
PRIMARY KEY (id)
);
By the way, MySQL (but not SQLite) allows comments on the table itself:
CREATE TABLE table1 (
id INT AUTO_INCREMENT,
val INT,
PRIMARY KEY (id)
) COMMENT = 'Table of stuff';
-- Show the comment
SHOW TABLE STATUS WHERE NAME='table1';
-- Just show names and comments
SELECT `TABLE_NAME`, `TABLE_COMMENT`
FROM information_schema.tables
WHERE table_schema = DATABASE();
I have 2 tables in SQLite. One is called contacts and the other is phoneNumbers. phoneNumbers is linked to contacts with integer references. Here is the tables:
CREATE TABLE contacts(
id INTEGER PRIMARY KEY,
name text
);
CREATE TABLE phoneNumbers(
id INTEGER PRIMARY KEY,
homePhone text,
contact_id INTEGER REFERENCES contacts(id)
);
My question is, how can I access all homePhone that is linked to contacts (id) 1?
Hope this is clear. If you have any questions, feel free to ask in the comments.
Here you have it:
SELECT homePhone
FROM phoneNumbers
JOIN contacts
ON homePhone.contact_id=contacts.id
WHERE contacts.id=1
And never use SELECT * :
https://stackoverflow.com/a/3639964/1919749
I'm working on a SQLite Database. The database is already filled, but I want to refactor it. Here is a sample of what I need to do:
I currently have one table:
CREATE TABLE Cars (ID INTEGER PRIMARY KEY,
Name VARCHAR(32),
TopSpeed FLOAT,
EngineCap FLOAT);
I want to split this into two tables:
CREATE TABLE Vehicles (ID INTEGER PRIMARY KEY,
Name VARCHAR(32),
TopSpeed FLOAT);
CREATE TABLE Cars (ID INTEGER PRIMARY KEY,
VehicleID INTEGER CONSTRAINT FK_Cars REFERENCES [Vehicles](ID),
EngineCap FLOAT);
I have figured out to create a temporary table with the Cars table contents, and I can fill up the Vehicles table with the contents of the Cars table:
CREATE TEMPORARY TABLE Cars_temp AS SELECT * FROM Cars;
INSERT INTO Vehicles (Name, TopSpeed)
SELECT Name, TopSpeed FROM Cars_temp;
But I am still looking for a way to go over that same selection, while putting the EngineCap field into the new Cars table and somehow extracting the corresponding ID value from the Vehicles table to put into the VehicleID foreign key field on the Cars table.
I'm open for workaround or alternative approaches.
Thanks.
Since #mateusza did not provide an example, I've made one:
Suppose you have this table:
CREATE TABLE [Customer] (
[name] TEXT,
[street] TEXT,
[city] TEXT);
Now you want to move street and city into a separate table Address, so you'll end up with two tables:
CREATE TABLE [Customer2] (
[name] TEXT,
[addr] INTEGER);
CREATE TABLE [Address] (
[rowid] INTEGER NOT NULL,
[street] TEXT,
[city] TEXT,
PRIMARY KEY ([rowid])
);
(For this example, I'm doing the conversion in the same database. You'd probably use two DBs, converting one into the other, with an SQL ATTACH command.)
Now we create a view (which imitates our original table using the new tables) and the trigger:
CREATE VIEW Customer1 (name, street, city) AS
SELECT C.name, A.street, A.city FROM Customer2 AS C
JOIN Address as A ON (C.addr == A.rowid);
CREATE TEMP TRIGGER TempTrig INSTEAD OF INSERT ON Customer1 FOR EACH ROW BEGIN
INSERT INTO Address (street, city) SELECT NEW.street, NEW.city;
INSERT INTO Customer2 (addr, name) SELECT last_insert_rowid(), NEW.name;
END;
Now you can copy the table rows:
INSERT INTO Customer1 (name, street, city) SELECT name, street, city FROM Customer;
The above is a simplified case where you'd only move some data into a single new table.
A more complex (and more general) case is where you want to...
Separate your original table's columns into several foreign tables, and
Have unique entries in the foreign tables (that's usually the reason why you'd refactor your table).
This adds some additional challenges:
You'll end up inserting into multiple tables before you can insert their rowids into the table with the referencing rowids. This requires storing the results of each INSERT's last_insert_rowid() into a temporary table.
If the value already exists in the foreign table, its rowid must be stored instead of the one from the (non-executed) insertion operation.
Here's a complete solution for this. It manages a database of music records, constisting of a song's name, album title and artist name.
-- Original table
CREATE TABLE [Song] (
[title] TEXT,
[album] TEXT,
[artist] TEXT
);
-- Refactored tables
CREATE TABLE [Song2] (
[title] TEXT,
[album_rowid] INTEGER,
[artist_rowid] INTEGER
);
CREATE TABLE [Album] (
[rowid] INTEGER PRIMARY KEY AUTOINCREMENT,
[title] TEXT UNIQUE
);
CREATE TABLE [Artist] (
[rowid] INTEGER PRIMARY KEY AUTOINCREMENT,
[name] TEXT UNIQUE
);
-- Fill with sample data
INSERT INTO Song VALUES ("Hunting Girl", "Songs From The Wood", "Jethro Tull");
INSERT INTO Song VALUES ("Acres Wild", "Heavy Horses", "Jethro Tull");
INSERT INTO Song VALUES ("Broadford Bazar", "Heavy Horses", "Jethro Tull");
INSERT INTO Song VALUES ("Statue of Liberty", "White Music", "XTC");
INSERT INTO Song VALUES ("Standing In For Joe", "Wasp Star", "XTC");
INSERT INTO Song VALUES ("Velvet Green", "Songs From The Wood", "Jethro Tull");
-- Conversion starts here
CREATE TEMP TABLE [TempRowIDs] (
[album_id] INTEGER,
[artist_id] INTEGER
);
CREATE VIEW Song1 (title, album, artist) AS
SELECT Song2.title, Album.title, Artist.name
FROM Song2
JOIN Album ON (Song2.album_rowid == Album.rowid)
JOIN Artist ON (Song2.artist_rowid == Artist.rowid);
CREATE TEMP TRIGGER TempTrig INSTEAD OF INSERT ON Song1 FOR EACH ROW BEGIN
INSERT OR IGNORE INTO Album (title) SELECT NEW.album;
UPDATE TempRowIDs SET album_id = (SELECT COALESCE (
(SELECT rowid FROM Album WHERE changes()==0 AND title==NEW.album), last_insert_rowid()
) ) WHERE rowid==1;
INSERT OR IGNORE INTO Artist (name) SELECT NEW.artist;
UPDATE TempRowIDs SET artist_id = (SELECT COALESCE (
(SELECT rowid FROM Artist WHERE changes()==0 AND name==NEW.artist), last_insert_rowid()
) ) WHERE rowid==1;
INSERT INTO Song2 (title, album_rowid, artist_rowid) SELECT
NEW.title, (SELECT album_id FROM TempRowIDs), (SELECT artist_id FROM TempRowIDs);
END;
INSERT INTO TempRowIDs DEFAULT VALUES;
INSERT INTO Song1 (title, album, artist) SELECT title, album, artist FROM Song;
DROP TRIGGER TempTrig;
DROP TABLE TempRowIDs;
-- Conversion ends here
-- Print results
SELECT * FROM Song;
SELECT * FROM Song1;
-- Check if original and copy are identical (https://stackoverflow.com/a/13865679/43615)
SELECT CASE WHEN (SELECT COUNT(*) FROM (SELECT * FROM Song UNION SELECT * FROM Song1)) == (SELECT COUNT() FROM Song) THEN 'Success' ELSE 'Failure' END;
Note that this example has one potential issue: If the constraints on the foreign table are more complex, the SELECT rowid FROM search for the existing entry needs to be updated accordingly. Ideally, SQLite should provide a way to determine the conflicting rowid somehow, but it doesn't, unfortunately (see this related question).
Simple solution without triggers:
create VEHICLES_TEMP table including the CAR_ID
create your new CARS table without the VEHICLES columns you don't want
update CARS with VEHICLE_ID taken from VEHICLES_TEMP (identified by the CAR_ID)
create final VEHICLES table without the CAR_ID
Create a table New_Cars and a INSTEAD OF INSERT trigger, which will insert data to both tables Vehicles and Cars. When inserting to Cars, you can use last_insert_rowid() function to refer to inserted row in Vehicles table.
This can be temporary solution, or you can leave it in your database for further modifications.