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atabases are an integral part of even the simplest modern applications. While most readers might tend to relate databases to websites and large corporate solutions, you might be surprised to know that you can also use one to store data managed within a Qt application. Qt offers a module for relational databases called QtSql. SQL, which stands for structured query language, is a language used for manipulating relational databases. Using SQL, you can communicate between different database engines and your application. Qt supports many of the most popular databases, including MySQL, Oracle, PostgreSQL, Sybase, DB2, SQLite, Interbase, and ODBC. These drivers can be built as plugins or can be integrated in Qt. In this chapter you ll learn how to integrate both the MySQL and SQLite databases with your Qt applications. You might use MySQL in more sophisticated situations and use SQLite when a database is handy but a full-featured database server might be considered overkill.
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A Quick Introduction to SQL
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Before you begin looking at some basic SQL statements, you should understand that SQL is another programming language. This book will not teach you to develop using SQL; it will show you only the very basic information. You also need to know that the different database servers supported by Qt support different dialects of SQL. This means that a statement can look slightly different for a MySQL connection when compared with an SQLite connection. By sticking to the very basics, these disparities can be avoided, but be prepared to read up on the database engine you choose to use in the SQL manuals. The statements used and described in this section have been tested with both MySQL and SQLite, so there will be no dialectal hiccups.
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Note An SQL statement is also known as a query because some statements are used to query the
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The rest of this chapter discusses relational databases, which are collections of tables. The tables each have a name and a set of columns and rows. The columns define the structure of
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CHAPTER 13 DATABASES
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the table, while the rows contain the data. The tables are then tied together through relations, in which column values from different tables are linked to each other. Each column has a name and a type, which make it possible to control what goes where and to retrieve by name. You can also control the allowed contents so that a NULL value will be replaced by a default, or you can disallow NULL values altogether. The rows contain the data as defined by the columns. When you are working with a database you usually search for rows, add them, update them, or remove them. The first thing you need to do is create an actual database, and the way you create it depends on the database server that you plan to use. Please refer to the user manual of your server for details. Before you can start adding rows, you need to create a table by using the CREATE TABLE command. Let s call the table names. The following statement creates a table with an integer column called id and two strings called firstname and lastname: CREATE TABLE names ( id INTEGER PRIMARY KEY, firstname VARCHAR(30), lastname VARCHAR(30) ) In the statement, you specify id as a PRIMARY KEY, which means that there can t be two identical id values in the same table. You can identify each row by its id column, which can be used when searching for data. The types used are INTEGER for integer values and VARCHAR(30) for the strings. The VARCHAR type is a string of variable length. The value inside the parentheses limits the length of the strings, so the firstname and lastname strings must be 30 characters or fewer. One problem with the statement is that if the table already exists, it will fail. You can solve this problem by adding the IF NOT EXISTS to it to create the following statement: CREATE TABLE IF NOT EXISTS names ( id INTEGER PRIMARY KEY, firstname VARCHAR(30), lastname VARCHAR(30) ) This statement adds the table or just passes by if it already exists. To remove a table, use the DROP TABLE command. To drop the names table you just created, simply execute the following: DROP TABLE names
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