SQL

We now provide an overview of SQL in the context of our books sample database. You will be able to use the SQL discussed here in the examples later in the chapter.

The SQL keywords listed in Fig. 25.12 are discussed in the context of complete SQL queries and statements in the next several subsections. Other SQL keywords are beyond the scope of this text. To learn other keywords, you could refer to the SQL reference guide supplied by the vendor of the RDBMS you are using. [Note: For more information on SQL, please refer to the Internet and Web resources in Section 25.12 and the Recommended Readings at the end of this chapter.]

25.4.1. Basic SELECT Query

Let us consider several SQL queries that extract information from database books. A SQL query "selects" rows and columns from one or more tables in a database. Such selections are performed by queries with the SELECT keyword. The basic form of a SELECT query is

   SELECT * FROM tableName

in which the asterisk (*) indicates that all columns from the tableName table should be retrieved. For example, to retrieve all the data in the authors table, use

SELECT * FROM authors

Most programs do not require all the data in a table. To retrieve only specific columns from a table, replace the asterisk (*) with a comma-separated list of the column names. For example, to retrieve only the columns authorID and lastName for all rows in the authors table, use the query

SELECT authorID, lastName FROM authors

This query returns the data listed in Fig. 25.13.

Software Engineering Observation 25.2

Common Programming Error 25.4

25.4.2. WHERE Clause

In most cases, it is necessary to locate rows in a database that satisfy certain selection criteria. Only rows that satisfy the selection criteria (formally called predicates) are selected. SQL uses the optional WHERE clause in a query to specify the selection criteria for the query. The basic form of a query with selection criteria is

   SELECT columnName1, columnName2, ... FROM tableName WHERE criteria

For example, to select the title, editionNumber and copyright columns from table titles for which the copyright date is greater than 2002, use the query

SELECT title, editionNumber, copyright FROM titles WHERE copyright > 2002

Figure 25.14 shows the result of the preceding query. The WHERE clause criteria can contain the operators <, >, <=, >=, =, <> and LIKE. Operator LIKE is used for pattern matching with wildcard characters percent (%) and underscore (_). Pattern matching allows SQL to search for strings that match a given pattern.

A pattern that contains a percent character (%) searches for strings that have zero or more characters at the percent character's position in the pattern. For example, the following query locates the rows of all the authors whose last name starts with the letter D:

SELECT authorID, firstName, lastName FROM authors WHERE lastName LIKE 'D%'

The preceding query selects the two rows shown in Fig. 25.15, because two of the four authors in our database have a last name starting with the letter D (followed by zero or more characters). The % in the WHERE clause's LIKE pattern indicates that any number of characters can appear after the letter D in the lastName column. Note that the pattern string is surrounded by single-quote characters.

Portability Tip 25.1

Portability Tip 25.2

Portability Tip 25.3

An underscore (_) in the pattern string indicates a single wildcard character at that position in the pattern. For example, the following query locates the rows of all the authors whose last names start with any character (specified by _), followed by the letter i, followed by any number of additional characters (specified by %):

SELECT authorID, firstName, lastName FROM authors WHERE lastName LIKE '_i%'

The preceding query produces the row shown in Fig. 25.16, because only one author in our database has a last name that contains the letter i as its second letter.

Portability Tip 25.4

25.4.3. ORDER BY Clause

The rows in the result of a query can be sorted into ascending or descending order by using the optional ORDER BY clause. The basic form of a query with an ORDER BY clause is

   SELECT columnName1, columnName2, ... FROM tableName ORDER BY column ASC

   SELECT columnName1, columnName2, ... FROM tableName ORDER BY column DESC

where ASC specifies ascending order (lowest to highest), DESC specifies descending order (highest to lowest) and column specifies the column on which the sort is based. For example, to obtain the list of authors in ascending order by last name (Fig. 25.17), use the query

SELECT authorID, firstName, lastName FROM authors ORDER BY lastName ASC

Note that the default sorting order is ascending, so ASC is optional. To obtain the same list of authors in descending order by last name (Fig. 25.18), use the query

SELECT authorID, firstName, lastName FROM authors ORDER BY lastName DESC

Multiple columns can be used for sorting with an ORDER BY clause of the form

  ORDER BY column1 sortingOrder, column2 sortingOrder, ...

where sortingOrder is either ASC or DESC. Note that the sortingOrder does not have to be identical for each column. The query

SELECT authorID, firstName, lastName FROM authors ORDER BY lastName, firstName

sorts all the rows in ascending order by last name, then by first name. If any rows have the same last name value, they are returned sorted by first name (Fig. 25.19).

The WHERE and ORDER BY clauses can be combined in one query. For example, the query

SELECT isbn, title, editionNumber, copyright, price FROM titles WHERE title LIKE '%How to Program' ORDER BY title ASC

returns the isbn, title, editionNumber, copyright and price of each book in the titles table that has a title ending with "How to Program" and sorts them in ascending order by title. A portion of the query results are shown in Fig. 25.20.

25.4.4. Merging Data from Multiple Tables: INNER JOIN

Database designers often split related data into separate tables to ensure that a database does not store data redundantly. For example, the books database has tables authors and titles. We use an authorISBN table to store the relationship data between authors and their corresponding titles. If we did not separate this information into individual tables, we would need to include author information with each entry in the titles table. This would result in the database storing duplicate author information for authors who wrote multiple books. Often, it is necessary to merge data from multiple tables into a single result. Referred to as joining the tables, this is specified by an INNER JOIN operator in the query. An INNER JOIN merges rows from two tables by matching values in columns that are common to the tables. The basic form of an INNER JOIN is:

   SELECT columnName1, columnName2,...

   FROM table1

   INNER JOIN table2

      ON table1. columnName = table2. columnName

The ON clause of the INNER JOIN specifies the columns from each table that are compared to determine which rows are merged. For example, the following query produces a list of authors accompanied by the ISBNs for books written by each author:

SELECT firstName, lastName, isbn FROM authors INNER JOIN authorISBN ON authors.authorID = authorISBN.authorID ORDER BY lastName, firstName

The query merges data from the firstName and lastName columns from table authors with the isbn column from table authorISBN, sorting the result in ascending order by lastName and firstName. Note the use of the syntax tableName. columnName in the ON clause. This syntax (called a qualified name) specifies the columns from each table that should be compared to join the tables. The "tableName." syntax is required if the columns have the same name in both tables. The same syntax can be used in any query to distinguish columns in different tables that have the same name. In some systems, table names qualified with the database name can be used to perform cross-database queries.

Software Engineering Observation 25.3

Common Programming Error 25.5

As always, the query can contain an ORDER BY clause. Figure 25.21 depicts a portion of the results of the preceding query, ordered by lastName and firstName. [Note: To save space, we split the result of the query into two columns, each containing the firstName, lastName and isbn columns.]

25.4.5. INSERT Statement

The INSERT statement inserts a row into a table. The basic form of this statement is

   INSERT INTO tableName ( columnName1, columnName2, ..., columnNameN )

      VALUES ( value1, value2, ..., valueN )

where tableName is the table in which to insert the row. The tableName is followed by a comma-separated list of column names in parentheses (this list is not required if the INSERT operation specifies a value for every column of the table in the correct order). The list of column names is followed by the SQL keyword VALUES and a comma-separated list of values in parentheses. The values specified here must match the columns specified after the table name in both order and type (se.g., if columnName1 is supposed to be the firstName column, then value1 should be a string in single quotes representing the first name). Always explicitly list the columns when inserting rows. If the order of the columns changes in the table, using only VALUES may cause an error. The INSERT statement

INSERT INTO authors (firstName, lastName) VALUES ( 'Sue', 'Smith' )

inserts a row into the authors table. The statement indicates that values are provided for the firstName and lastName columns. The corresponding values are 'Sue' and 'Smith'. We do not specify an authorID in this example because authorID is an autoincremented column in the authors table. For every row added to this table, MySQL assigns a unique authorID value that is the next value in the autoincremented sequence (i.e., 1, 2, 3 and so on). In this case, Sue Smith would be assigned authorID number 5. Figure 25.22 shows the authors table after the INSERT operation. [Note: Not every database management system supports autoincremented columns. Check the documentation for your DBMS for alternatives to autoincremented columns.]

Common Programming Error 25.6

Common Programming Error 25.7

25.4.6. UPDATE Statement

An UPDATE statement modifies data in a table. The basic form of the UPDATE statement is

   UPDATE tableName

      SET columnName1 = value1, columnName2 = value2, ..., columnNameN = valueN

      WHERE criteria

where tableName is the table to update. The tableName is followed by keyword SET and a comma-separated list of column name-value pairs in the format columnName = value. The optional WHERE clause provides criteria that determine which rows to update. Though not required, the WHERE clause is typically used, unless a change is to be made to every row. The UPDATE statement

UPDATE authors SET lastName = 'Jones' WHERE lastName = 'Smith' AND firstName = 'Sue'

updates a row in the authors table. The statement indicates that lastName will be assigned the value Jones for the row in which lastName is equal to Smith and firstName is equal to Sue. [Note: If there are multiple rows with the first name "Sue" and the last name "Smith," this statement will modify all such rows to have the last name "Jones."] If we know the authorID in advance of the UPDATE operation (possibly because we searched for it previously), the WHERE clause could be simplified as follows:

WHERE AuthorID = 5

Figure 25.23 shows the authors table after the UPDATE operation has taken place.

25.4.7. DELETE Statement

A SQL DELETE statement removes rows from a table. The basic form of a DELETE statement is

   DELETE FROM tableName WHERE criteria

where tableName is the table from which to delete. The optional WHERE clause specifies the criteria used to determine which rows to delete. The DELETE statement

DELETE FROM authors WHERE lastName = 'Jones' AND firstName = 'Sue'

deletes the row for Sue Jones in the authors table. If we know the authorID in advance of the DELETE operation, the WHERE clause can be simplified as follows:

WHERE authorID = 5

Figure 25.24 shows the authors table after the DELETE operation has taken place.