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The most common use of databases (especially within the context of Rails) is to implement something we call CRUD functionality: create, read, update, and delete. Corresponding to the CRUD components are the most commonly used SQL commands INSERT, SELECT, UPDATE, and DELETE, as shown in Table D-1.
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APPENDIX B DATABASES 101
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Table D-1. Common SQL Commands Operation
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Create Read Update Delete
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SQL Command
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INSERT SELECT UPDATE DELETE
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We ll use the articles table presented in the previous section to show you some examples of how these commands work. Remember that it s not necessary for you to have a complete understanding of SQL to work with Rails. After all, the whole point of Active Record is to alleviate the tedium of needing to construct complex SQL statements to view and otherwise manipulate your data.
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Selecting Data
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The SELECT statement is a powerful and useful SQL command. Using SELECT, you can query (or request information from) the database and mine it for information. You can also give SELECT any number conditions, a limit to the number of rows it returns, and instructions on how to order its results. Earlier, we used the SELECT statement to see the data in the articles table:
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SELECT * FROM articles;
The asterisk (*) character is a wildcard that means every column. This statement is like saying, Show me the values in every column for every row. This is the easiest way to look at the contents of a table. But it s not too often that you need to see every single row, and for tables with a lot of data, you could end up with a really large list. So, sometimes it s not very efficient to select everything. Fortunately, you can also select specific columns by name. For example, to select only the title column, we would do this:
SELECT title FROM articles;
+-----------------+ | title | +-----------------+ | Beginning Rails | +-----------------+ 1 row in set (0.00 sec)
APPENDIX B DATABASES 101
Instead of returning all fields, it returns only the one requested: title. To return both the title and the author fields, just add author to the list of columns to select.
SELECT title, author FROM articles;
+-----------------+--------------+ | title | author | +-----------------+--------------+ | Beginning Rails | Packagethief | +-----------------+--------------+ 1 row in set (0.00 sec)
In both cases, the command returns all rows. If there were 100 rows in the table, they would all be returned. But what about when you need to find a particular row This is where conditions come in to play. To supply conditions to a SELECT statement, you use the WHERE clause.
SELECT fields FROM table WHERE some_field = some_value;
Let s apply this to the articles table by finding a row by its primary key.
SELECT * FROM articles WHERE id = 1;
+----+-----------------+--------------+ | id | title | author | +----+-----------------+--------------+ | 1 | Beginning Rails | Packagethief | +----+-----------------+--------------+
This query returns only the row whose primary key, id, matches the condition. You can use this technique on any field id, title, or author or all of these combined. Conditions can be chained together using AND and further modified using OR. For example, the following query returns only records whose titles and authors match the specified criteria.
SELECT * FROM articles WHERE title = 'Beginning Rails' AND author = 'Packagethief';
APPENDIX B DATABASES 101
Inserting Data
To insert a row into a table, you use the INSERT command. INSERT requires a table name, a list of fields, and a list of values to insert into those fields. Here s a basic INSERT statement for the articles table:
INSERT INTO articles (title, author) VALUES ('Intro to SQL', 'ccjr');
Query OK, 1 row affected (0.01 sec)
The response tells us that our command was successful and that it affected one row. To see what was inserted, we again use the SELECT command.
SELECT * FROM articles;
+----+-----------------+--------------+ | id | title | author | +----+-----------------+--------------+ | 1 | Beginning Rails | Packagethief | | 2 | Intro to SQL | ccjr | +----+-----------------+--------------+
Great! We now have two rows in our table. Notice that in our INSERT statement, we didn t specify the id field. That s because, as you ll recall, it s handled automatically by the database. If we were to insert a value ourselves, we wouldn t have a reliable way to guarantee its uniqueness and could cause an error if we attempted to insert a duplicate value. MySQL has automatically inserted an id of 2 into the field.
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