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Relational databases aren't like programming languages. One problem I've seen with applications is when engineers attempt to use a database as though it were a programming language, creating DROP TABLE IF EXISTS files; CREATE TABLE files ( id MEDIUMINT, name TEXT, path TEXT ); DROP TABLE IF EXISTS users; CREATE TABLE users ( id MEDIUMINT, login TEXT, password TEXT, files TEXT ); INSERT INTO files VALUES ( 1, 'test1.jpg', 'media/test1.jpg' ); INSERT INTO files VALUES ( 2, 'test1.jpg', 'media/test1.jpg' ); INSERT INTO users VALUES ( 1, 'jack', 'pass', '1,2' ); One user in the system can have multiple files.
In a programming language, you would use an array to represent the files associated with a user.
The latter approach is susceptible to race conditions on high-volume sites.
Once in a while, we see an application in which each table is in a separate database. Files.sql: CREATE TABLE files ( id MEDIUMINT, user_id MEDIUMINT, name TEXT, path TEXT ); Load_files.sql: INSERT INTO files VALUES ( 1, 1, 'test1.jpg', 'files/test1.jpg' ); INSERT INTO files VALUES ( 2, 1, 'test2.jpg', 'files/test2.jpg' ); Users.sql: DROP TABLE IF EXISTS users; CREATE TABLE users ( id MEDIUMINT, login TEXT, password TEXT ); Load_users.sql: INSERT INTO users VALUES ( 1, 'jack', 'pass' ); INSERT INTO users VALUES ( 2, 'jon', 'pass' ); function connects to the files table and retrieves the file rows associated with the given user.
Instead of performing two queries, we're performing one.
While this problem sounds a bit far-fetched, we've seen it in practice enough times to know that all tables should be in the same database unless there's a pressing reason otherwise. Instead, they use relations among tables to create a one-to-many structure between objects, which has the same effect as an array.
There are reasons for doing that in extraordinarily large databases, but for an average application, you don't need this level of segmentation. The syntax may or may not work between different database engines. A better way to do all this is to load the data into one database, then perform a query, such as that shown below.
The objective of this chapter is to develop an inventory of ITS projects on the U. These projects include ITS studies and deployments.
Both provide abstraction from the choice of a particular database.
Therefore, your code can run without too much adjustment on IBM® DB2®, My SQL, Postgre SQL, or any other database you want to connect to.
The code isn't complex, and it uses the database as it was intended.
I can't tell you how many times we've seen large applications in which the code first retrieves a list of entities -- say, customers -- then comes back and retrieves them one by one to get the details for each entity. Look at how many queries we had to perform to retrieve only the books by Jack Herrington.The advantage of using this method is that it will work across different database systems.