FIGURE 18.3 You can use three related tables to track customers, orders, and outstanding invoices.
Now that you have an idea of how data is stored in an RDBMS and how the RDBMS structure enables you to work with that data, you are ready to learn how to input and output data from the database. This is where SQL comes in.
Understanding SQL Basics
SQL (pronounced "S-Q-L") is a database query language understood by virtually all RDBMSs available today. You use SQL statements to get data into and retrieve data from a database. As with statements in any language, SQL statements have a defined structure that determines their meanings and functions.
As a DBA, you should understand the basics of SQL, even if you will not be doing any of the actual programming yourself. Fortunately, SQL is similar to standard English, so learning the basics is simple.
As mentioned previously, an RDBMS stores data in tables that look similar to spread sheets. Of course, before you can store any data in a database, you need to create the necessary tables and columns to store the data. You do this by using the CREATE statement. For example, the cd_collection table from Figure 18.2 has five columns, or fields: id, title, artist, year, and rating.
SQL provides several column types for data that define what kind of data will be stored in the column. Some of the available types are INT, FLOAT, CHAR, and VARCHAR. Both CHAR and VARCHAR hold text strings, with the difference being that CHAR holds a fixed-length string, whereas VARCHAR holds a variable-length string.
There are also special column types, such as DATE, which takes data in only a date format, and ENUMs (enumerations), which can be used to specify that only certain values are allowed. If, for example, you wanted to record the genres of your CDs, you could use an ENUM column that accepts only the values POP, ROCK, EASY_LISTENING, and so on. You will learn more about ENUM later in this chapter.
Looking at the cd_collection table, you can see that three of the columns hold numerical data and the other two hold string data. In addition, the character strings are of variable length. Based on this information, you can discern that the best type to use for the text columns is type VARCHAR, and the best type to use for the others is INT. You should notice something else about the cd_collection table: One of the CDs is missing a rating, perhaps because we have not listened to it yet. This value, therefore, is optional; it starts empty and can be filled in later.
You are now ready to create a table. As mentioned before, you do this by using the CREATE statement, which uses the following syntax:
CREATE TABLE table_name (column_name column_type(parameters) options, ...);
You should know the following about the CREATE statement:
► SQL commands are not case sensitive — For example, CREATE TABLE, create table, and Create Table are all valid.
► Whitespace is generally ignored — This means you should use it to make your SQL commands clearer.
The following example shows how to create the table for the cd_collection database:
CREATE TABLE cd_collection
(
id INT NOT NULL,
title VARCHAR(50) NOT NULL,
artist VARCHAR(50) NOT NULL,
year VARCHAR(50) NOT NULL,
rating VARCHAR(50) NULL
);
Notice that the statement terminates with a semicolon. This is how SQL knows you are finished with all the entries in the statement. In some cases, the semicolon can be omitted, and we will point out these cases when they arise.
SQL has a number of reserved keywords that cannot be used in table names or field names. For example, if you keep track of CDs you want to take with you on vacation, you would not be able to use the field name select because that is a reserved keyword. Instead, you should either choose a different name (selected?) or just prefix the field name with an f, such as fselect.
After you create the tables, you can put data into them. You can insert data manually with the INSERT statement, which uses the following syntax:
INSERT INTO table_name VALUES('value1' , 'value2', 'value3', ...);
This statement inserts value1, value2, and so on into the table table_name. The values that are inserted constitute one row, or record, in the database. Unless specified otherwise, values are inserted in the order in which the columns are listed in the database table. If, for some reason, you want to insert values in a different order (or if you want to insert only a few values and they are not in sequential order), you can specify in which columns you want the data to go by using the following syntax:
INSERT INTO table_name (column1,column4) VALUES('value1', 'value2');
You can also fill multiple rows with a single INSERT statement, using syntax such as the following:
INSERT INTO table_name VALUES('value1', 'value2'),('value3', 'value4');
In this statement, value1 and value2 are inserted into the first row and values and value4 are inserted into the second row.
The following example shows how you would insert the Nevermind entry into the cd_collection table:
INSERT INTO cd_collection VALUES(9, 'Nevermind', ''Nirvana', '1991, ''NULL);
MySQL requires the NULL value for the last column (rating) if you do not want to include a rating. PostgreSQL, on the other hand, lets you get away with just omitting the last column. Of course, if you had columns in the middle that were null, you would need to explicitly state NULL in the INSERT statement.
Normally, INSERT statements are coded into a front-end program so that users adding data to the database do not have to worry about the SQL statements involved.
Of course, the main reason for storing data in a database is so that you can later look up, sort, and generate reports on that data. Basic data retrieval is done with the SELECT statement, which has the following syntax: