SQL

We now overview SQL in the context of the Books database. Later in the chapter, you will build C# applications that execute SQL queries and access their results using ADO.NET technology. Though the Visual C# IDE provides visual tools that hide some of the SQL used to manipulate databases, it is nevertheless important to understand SQL basics. Knowing the types of operations you can perform will help you develop more advanced database-intensive applications.

Figure 20.10 lists some common SQL keywords used to form complete SQL statementswe discuss these keywords in the next several subsections. Other SQL keywords exist, but they are beyond the scope of this text. For additional information on SQL, please refer to the URLs listed in Section 20.11, Web Resources.

20.4.1. Basic SELECT Query

Let us consider several SQL queries that retrieve 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 the columns from the tableName table should be retrieved. For example, to retrieve all the data in the Authors table, use

SELECT * FROM Authors

Note that the rows of the Authors table are not guaranteed to be returned in any particular order. You will learn how to specify criteria for sorting rows in Section 20.4.3.

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 the rows in the Authors table, use the query

SELECT AuthorID, LastName FROM Authors

This query returns only the data listed in Fig. 20.11.

20.4.2. WHERE Clause

When users search a database for rows that satisfy certain selection criteria (formally called predicates), only rows that satisfy the selection criteria 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 more recent than 2004, use the query

SELECT Title, EditionNumber, Copyright FROM Titles WHERE Copyright > '2004'

Figure 20.12 shows the result of the preceding query.

The WHERE clause criteria can contain the relational operators <, >, <=, >=, = (equality), <> (inequality) and LIKE, as well as the logical operators AND, OR and NOT (discussed in Section 20.4.6). 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 names start with the letter D:

SELECT AuthorID, FirstName, LastName FROM Authors WHERE LastName LIKE 'D%'

The preceding query selects the two rows shown in Fig. 20.13, 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.

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 h, followed by any number of additional characters (specified by %):

SELECT AuthorID, FirstName, LastName FROM Authors WHERE LastName LIKE '_h%'

The preceding query produces the row shown in Fig. 20.14, because only one author in our database has a last name that contains the letter h as its second letter.

20.4.3. 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. 20.15), use the query

SELECT AuthorID, FirstName, LastName FROM Authors ORDER BY LastName ASC

The default sorting order is ascending, so ASC is optional in the preceding query.

To obtain the same list of authors in descending order by last name (Fig. 20.16), 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. For example, the query

SELECT Title, EditionNumber, Copyright FROM Titles ORDER BY Copyright DESC, Title ASC

returns the rows of the Titles table sorted first in descending order by copyright date, then in ascending order by title (Fig. 20.17). This means that rows with higher Copyright values are returned before rows with lower Copyright values, and any rows that have the same Copyright values are sorted in ascending order by title.

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

SELECT ISBN, Title, EditionNumber, Copyright FROM Titles WHERE Title LIKE '%How to Program' ORDER BY Title ASC

returns the ISBN, Title, EditionNumber and Copyright of each book in the Titles table that has a Title ending with "How to Program" and sorts them in ascending order by Title. The query results are shown in Fig. 20.18.

20.4.4. Merging Data from Multiple Tables: INNER JOIN

Database designers typically normalize databasesi.e., split related data into separate tables to ensure that a database does not store redundant data. For example, the Books database has tables Authors and Titles. We use an AuthorISBN table to store "links" between authors and 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 more than one book.

Often, it is desirable to merge data from multiple tables into a single result. This is referred to as joining the tables, and is specified by an INNER JOIN operator in the query. An INNER JOIN merges rows from two tables by testing for matching values in a column that is 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 combines the FirstName and LastName columns from table Authors and the ISBN column from table AuthorISBN, sorting the results 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 that have the same name in different tables.

As always, the query can contain an ORDER BY clause. Figure 20.19 depicts the results of the preceding query, ordered by LastName and FirstName.

20.4.5. 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 up with the columns specified after the table name in both order and type (e.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 rowsif the order of the columns in the table changes, 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 the values 'Sue' and 'Smith' are provided for the FirstName and LastName columns, respectively.

We do not specify an AuthorID in this example because AuthorID is an identity column in the Authors table (see Fig. 20.3). For every row added to this table, SQL Server assigns a unique AuthorID value that is the next value in an autoincremented sequence (i.e., 1, 2, 3 and so on). In this case, Sue Smith would be assigned AuthorID number 5. Figure 20.20 shows the Authors table after the INSERT operation. Not every DBMS supports identity or autoincremented columns.

20.4.6. 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. Keyword AND is a logical operator that, like the C# && operator, returns true if and only if both of its operands are true. Thus, the preceding statement assigns to LastName 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 modifies all such rows to have the last name "Jones."] Figure 20.21 shows the Authors table after the UPDATE operation has taken place. SQL also provides other logical operators, such as OR and NOT, which behave like their C# counterparts.

20.4.7. DELETE Statement

A 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. DELETE statements can delete multiple rows if the rows all meet the criteria in the WHERE clause. Figure 20.22 shows the Authors table after the DELETE operation has taken place.

SQL Wrap-Up

This concludes our SQL introduction. We demonstrated several commonly used SQL keywords, formed SQL queries that retrieved data from databases and formed other SQL statements that manipulated data in a database. Next, we introduce the ADO.NET object model, which allows C# applications to interact with databases. As you will see, ADO.NET objects manipulate databases using SQL statements like those presented here.