Книга: Fedora™ Unleashed, 2008 edition

Understanding SQL Basics

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.

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:

SELECT column1, column2, column3 FROM table_name WHERE search_criteria;

The first two parts of the statement — the SELECT and FROM parts —are required. The WHERE portion of the statement is optional. If it is omitted, all rows in the table table_name are returned.

The column1, column2, column3 indicates the name of the columns you want to see. If you want to see all columns, you can also use the wildcard * to show all the columns that match the search criteria. For example, the following statement displays all columns from the cd_collection table:

SELECT * FROM cd_collection;

If you wanted to see only the titles of all the CDs in the table, you would use a statement such as the following:

SELECT title FROM cd_collection;

To select the title and year of a CD, you would use the following:

SELECT title, year FROM cd_collection;

If you want something a little fancier, you can use SQL to print the CD title followed by the year in parentheses, as is the convention. Both MySQL and PostgreSQL provide string concatenation functions to handle problems such as this. However, the syntax is different in the two systems.

In MySQL, you can use the CONCAT() function to combine the title and year columns into one output column, along with parentheses. The following statement is an example:

SELECT CONCAT(title," (",year, ")") AS TitleYear FROM cd_collection;

That statement lists both the title and year under one column that has the label TitleYear. Note that there are two strings in the CONCAT() function along with the fields — these add whitespace and the parentheses.

In PostgreSQL, the string concatenation function is simply a double pipe (||). The following command is the PostgreSQL equivalent of the preceding MySQL command:

SELECT (genus||'' ('||species||')') AS TitleYear FROM cd_collection;

Note that the parentheses are optional, but they make the statement easier to read. Once again, the strings in the middle and at the end (note the space between the quotes) are used to insert spacing and parentheses between the title and year.

Of course, more often than not, you do not want a list of every single row in the data base. Rather, you want to find only rows that match certain characteristics. For this, you add the WHERE statement to the SELECT statement. For example, suppose that you want to find all the CDs in the cd_collection table that have a rating of 5. You would use a statement like the following:

SELECT * FROM cd_collection WHERE rating = 5;

Using the table from Figure 18.2, you can see that this query would return the rows for Trouser Jazz, Life for Rent, and The Two Towers. This is a simple query, and SQL is capable of handling queries much more complex than this. Complex queries can be written using logical AND and logical OR statements. For example, suppose that you want to refine the query so that it lists only those CDs that were not released in 2003. You would use a query like the following:

SELECT * FROM cd_collection WHERE rating = 5 AND year != 2003;

In SQL, != means "is not equal to." Once again looking at the table from Figure 18.2, you can see that this query returns the rows for Trouser Jazz and The Two Towers but doesn't return the row for Life for Rent because it was released in 2003.

So, what if you want to list all the CDs that have a rating of 3 or 4 except those released in the year 2000? This time, you combine logical AND and logical OR statements:

SELECT * FROM cd_collection WHERE rating = 3 OR rating = 4 AND year != 2000;

This query would return entries for Mind Bomb, Natural Elements, and Combat Rock. However, it wouldn't return entries for Adiemus 4 because it was released in 2000.

SQL is capable of far more than is demonstrated here. But as mentioned before, this section is not intended to teach you all there is to know about SQL programming; rather, it teaches you the basics so that you can be a more effective DBA.

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