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SQL in a Nutshell: A Desktop Quick Reference

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SQL in a Nutshell applies the eminently useful "Nutshell" format to Structured Query Language (SQL), the elegant--but complex--descriptive language that is used to create and manipulate large stores of data. For SQL programmers, analysts, and database administrators, the new second edition of SQL in a Nutshell is the essential date language reference for the world's top SQL database products. SQL in a Nutshell is a lean, focused, and thoroughly comprehensive reference for those who live in a deadline-driven world.This invaluable desktop quick reference drills down and documents every SQL command and how to use it in both commercial (Oracle, DB2, and Microsoft SQL Server) and open source implementations (PostgreSQL, and MySQL). It describes every command and reference and includes the command syntax (by vendor, if the syntax differs across implementations), a clear description, and practical examples that illustrate important concepts and uses. And it also explains how the leading commercial and open sources database product implement SQL. This wealth of information is packed into a succinct, comprehensive, and extraordinarily easy-to-use format that covers the SQL syntax of no less than 4 different databases.When you need fast, accurate, detailed, and up-to-date SQL information, SQL in a Nutshell, Second Edition will be the quick reference you'll reach for every time. SQL in a Nutshell is small enough to keep by your keyboard, and concise (as well as clearly organized) enough that you can look up the syntax you need quickly without having to wade through a lot of useless fluff. You won't want to work on a project involving SQL without it.

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1.1. The Relational Model and ANSI SQL

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Relational Database Management Systems (RDBMSs), such as those covered in this book, are the primary engines of information systems worldwide, particularly web applications and distributed client/server computing systems. They enable a multitude of users to quickly and simultaneously access, create, edit, and manipulate data without impeding or impacting other users. They also allow developers to write useful applications to access their resources as well as provide administrators with the capabilities they need to maintain, secure, and optimize organizational data resources.

An RDBMS is defined as a system whose users view data as a collection of tables related to each other through common data values. Data is stored in tables, which are composed of rows and columns. Tables of independent data can be linked (or related) to one another if they each have unique, identifying columns of data (called keys) that represent a data value held in common. E. F. Codd first described relational database theory in his landmark paper "A Relational Model of Data for Large Shared Data Banks," published in the Communications of the ACM (Association for Computing Machinery) in June, 1970. Under Codd's new relational data model, data was structured (into tables of rows and columns); manageable using operations like selections, projections, and joins; and consistent as the result of integrity rules like keys and referential integrity. Codd also articulated rules that governed how a relational database should be designed; the process for applying these rules is now known as normalization.

 

1.2. History of the SQL Standard

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SQL99 had two main parts, Foundation:1999 and Bindings:1999. The SQL2003 Foundation section includes all of the Foundation and Bindings standards from SQL99, but a new section called Schemata was created.

The Core requirements of SQL2003 did not change from Core SQL99. So the database platforms that conformed to Core SQL99 automatically conform to SQL2003. Although the Core of SQL2003 has no additions (except for a few new reserved words), a number of individual statements and behaviors have been updated or modified. Because these updates are reflected in the individual syntax descriptions of each statement in Chapter 3, we won't spend time on them here.

A few elements of the Core in SQL99 have been deleted from SQL2003, including:

The BIT and BIT VARYING datatypes

The UNION JOIN clause

The UPDATE...SET ROW statement

In addition, a number of features, most of which were rather obscure, have been added, deleted, or renamed. At this point in time, many of the new features of SQL2003 standard are interesting mostly from an academic standpoint because none of the database platforms support the feature yet. However, there are a few new features that hold more than passing interest:

 

1.3. SQL Dialects

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The constantly evolving nature of the SQL standard has given rise to a number of SQL dialects among the various vendors and platforms. These dialects most commonly evolved because the user community of a given database vendor required capabilities in the database before the ANSI committee created a standard. Occasionally, though, a new feature is introduced by the academic or research communities due to competitive pressures from competing technologies. For example, many database vendors are augmenting their current programmatic offerings with Java (as is the case with DB2, Oracle, and Sybase) or VBScript (as Microsoft is doing). In the future, programmers and developers will use these programming languages in concert with SQL to build SQL programs.

Many of these dialects include conditional processing capabilities (such as that controlled through IF . . . THEN statements), control-of-flow functions (such as WHILE loops), variables, and error handling. Because ANSI had not yet developed a standard for these important features at the time users began to demand them, RDBMS developers and vendors were free to create their own commands and syntax. In fact, some of the earliest vendors from the 1980s have variances in the most elementary commands, such as SELECT, because their implementations predate the standards. ANSI is now refining standards that address these inconsistencies.

 

2.1. Database Platforms Described in This Book

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SQL in a Nutshell, Second Edition, describes the SQL standard and the platform-specific implementations of five leading RDBMSs:

IBM's popular DBMS runs on hardware platforms ranging from PCs all the way up to huge mainframes. It runs on a multitude of operating systems, including the burgeoning Linux OS. DB2 is very popular in corporate environments, especially those with a healthy investment in IBM software, equipment, or services. This book covers DB2 Universal Database Version 8.1 for Linux, Unix, and Windows.

MySQL is a popular open source DBMS that is known for its blistering performance. It runs on numerous operating systems, including most Linux variants. To improve performance, it has a slimmer feature set than do many other DBMSs. This book covers MySQL 4.0.

Oracle is a leading RDBMS in the commercial sector. It runs on a multitude of operating systems and hardware platforms. Its scalable, reliable architecture has made it the platform of choice for many users. SQL in a Nutshell, Second Edition, covers Oracle Database 10g.

 

2.2. Categories of Syntax

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To begin to use SQL, you must understand how statements are written. SQL syntax falls into four main categories. Each category is introduced in the following list and then explained in further detail in the sections that follow:

Describe a user- or system-supplied name for database objects, such as a database, a table, a constraint on a table, the columns in a table, a view, etc.

Describe a user- or system-supplied string or value that is not otherwise an identifier or a keyword. Literals may be strings like "hello", numbers like 1234, dates like "Jan 01, 2002", or Boolean values like TRUE.

Are symbols specifying an action performed on one or more expressions, most often in DELETE , INSERT, SELECT, or UPDATE statements. Operators are also used frequently in the creation of database objects.

Have special meaning to the database SQL parser, such as SELECT, GRANT, DELETE, or CREATE. Reserved words are words that cannot be used as identifiers within the database platform, usually commands or SQL statements. Keywords are words that may become reserved some time in the future. You can circumvent the restriction on using reserved words and keywords as identifiers through quoted identifiers, described in a moment. This is not recommended! Elsewhere in the book (besides here), we use the term keyword to describe both concepts.

 

2.3. SQL2003 and Platform-Specific Datatypes

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A table can contain one or many columns. Each column must be defined with a datatype that provides a general classification of the data that the column will store. In real-world applications, datatypes provide some control and efficiency as to how tables are defined and how the data is stored within the table. Using specific datatypes enables better, more understandable queries and helps control the integrity of data.

The tricky thing about SQL2003 datatypes is that they do not always map directly to an identical implementation in a given platform. Although the platforms specify "datatypes" that correspond to the SQL2003 datatypes, these are not always true SQL2003 datatypes. For example, MySQL's implementation of a BIT datatype is actually identical to a CHAR(1) datatype value. Nonetheless, each of the platform datatypes is close enough to the standard to be both easily understandable and job-ready.

The official SQL2003 datatypes (as opposed to platform-specific datatypes) fall into the general categories described in Table 2-8. (Note that the SQL2003 standard contains a few rarely used datatypes (ARRAY, MULTISET, REF, and ROW) that are shown only in Table 2-8 and not elsewhere in the book.)

 

2.4. Constraints

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Constraints are defined when you create or alter a table. The general syntax for constraints is shown here:

The syntax elements are as follows:

Begins a constraint definition and, optionally, provides a name for the constraint. When you omit constraint_name, the system will create one for you automatically. On some platforms, like DB2, you may omit the CONSTRAINT keyword as well. System-generated names are often incomprehensible. It is good practice to specify human-readable, sensible names for constraints.

Declares the constraint as one of the allowable types: CHECK, PRIMARY KEY, UNIQUE, or FOREIGN KEY. More information about each type of constraint appears later in this section.

Associates one or more columns with the constraint. Specify the columns in a comma-delimited list. The column list must be enclosed in parentheses. The column list should be omitted for column-level constraints. Columns are not used in every constraint. For example, CHECK constraints do not generally use column references.

 

3.1. How to Use This Chapter

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When researching a command in this chapter:

Read Section 3.2.

Check the platform support table.

Read the section on SQL2003 syntax and description even if you are looking for a specific platform implementation.

Finally, read the specific platform implementation information.

Any common features between the platform implementations of a command are discussed and compared against the SQL2003 topic. Thus, the subsection on a platform's implementation of a particular command may not describe every aspect of that command, since some of its details may be covered in the SQL2003 section. Please note that if there is a keyword that appears in a command syntax but not in its keyword description, this is because we chose not to repeat descriptions that appear under the ANSI entry.

If you skip straight to a platform implementation and do not see a discussion of a keyword or clause that you are interested in, that means it is described (in common across all supporting platforms) under the SQL2003 entry of the command.

 

3.2. SQL Platform Support

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Table 3-1 provides a listing of the SQL statements, the platforms that support them, and the degree to which they support them. The following list offers useful tips for reading Table 3-1, as well as what each abbreviation stands for. The sections that follow describe the commands from the table in detail:

The first column contains the alphabetized SQL commands.

The SQL statement class for each command is indicated in the second column.

The subsequent columns list the vendor's level of support:

The platform supports the SQL2003 standard for the particular command.

The platform supports the SQL2003 standard for the particular command, using vendor-specific code or syntax.

The platform supports some but not all of the functions specified by the SQL2003 standard for the particular command.

The platform does not support the particular command according to the SQL2003 standard.

Remember that even if a specific SQL2003 command is listed as "Not supported," the platform usually has alternative coding or syntax to enact the same command or function. Therefore, be sure to read the discussion and examples for each command later in this chapter. Likewise, a few of the commands in Table 3-1 are not found in the SQL2003 standard, and these have been indicated with the term "Non-ANSI" under the heading "SQL2003 class" in the table.

 

3.3. SQL Command Reference

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ALL/ANY/SOME Operators

The ALL operator performs a Boolean test of a subquery for the existence of a value in all rows. The ANY operator, and its synonym SOME, performs a Boolean test of a subquery for the existence of a value in any of the rows tested.

Platform

Command

DB2

Supported

MySQL

Not supported

Oracle

Supported

PostgreSQL

Supported

SQL Server

Supported

Tests a scalar expression, such as a column, against every value in the subquery, for ALL, and against every value until a match is found for ANY and SOME. All rows must match the expression to return a Boolean TRUE value for the ALL operator, while one or more rows must match the expression, for ANY and SOME, to return a Boolean TRUE value.

Compares the expression to the subquery. The comparison must be a standard comparison operator like =, <>, !=, >, >=, <, or <=.

The ALL operator returns a Boolean TRUE value when one of two things happen: either the subquery returns an empty set (i.e., no records) or every record in the set meets the comparison. ALL returns FALSE when any records in the set do not match the value comparison. ANY and SOME operators return a Boolean TRUE when at least one record in the subquery matches the comparison operation, and FALSE when no record matches the comparison operation (or when a subquery returns an empty result set). If even one return value of the subquery is NULL, then the operation evaluates as NULL, not as TRUE.

 

4.1. Types of Functions

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4.2. ANSI SQL Aggregate Functions

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Aggregate functions return a single value based upon a set of other values. If used among other expressions in the item list of a SELECT statement, the SELECT must have a GROUP BY or HAVING clause. No GROUP BY or HAVING clause is required if the aggregate function is the only value retrieved by the SELECT statement. The supported aggregate functions and their syntax are listed in Table 4-1.

Table4-1.ANSI SQL aggregate functions

Function

Usage

Computes the average value of a column given by expression.

Computes a correlation coefficient.

Counts the rows defined by the expression.

Counts all rows in the specified table or view.

Computes population covariance.

Computes sample covariance.

Computes the relative rank of a hypothetical row within a group of rows, where the rank is equal to the number of rows less than or equal to the hypothetical row divided by the number of rows in the group.

Generates a dense rank (no ranks are skipped) for a hypothetical row (value_list) in a group of rows generated by GROUP BY.

 

4.3. ANSI SQL Window Functions

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SQL2003 allows for a window_clause in aggregate function calls, the addition of which makes those functions into window functions. Both Oracle and DB2 support this window function syntax. This section describes how to use the window_clause within Oracle and DB2.

Oracle tends to refer to window functions as analytic functions.

Window, or analytic, functions are similar to standard aggregate functions in that they operate on multiple rows, or groups of rows, within the result set returned from a query. However, the groups of rows that a window function operates on are defined not by a GROUP BY clause, but by partitioning and windowing clauses. Furthermore, the order within these groups is defined by an ordering clause, but that order only affects function evaluation, and has no effect on the order in which rows are returned by the query.

Window functions are the last items in a query to be evaluated except for the ORDER BY clause. Because of this late evaluation, window functions cannot be used within the WHERE, GROUP BY, or HAVING clauses.

 

4.4. ANSI SQL Scalar Functions

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The ANSI SQL scalar functions return a single value each time they are invoked. The SQL standard provides many scalar functions that can be used to manipulate date and time types, strings, and numbers, as well as retrieve system information such as the current user or login name. Scalar functions fall into the categories listed in Table 4-2.

Table4-2.Categories of scalar functions

Function category

Explanation

Built-in

Performs operations on values or settings built into the database.

Oracle uses the term "built-in" to describe all the specialty functions that are provided by Oracle, and thus "built into" their DBMS. This is a distinct and separate usage from the "built-in" functions described here.

CASE and CAST

While these two functions operate on scalar input values, they are in a category all their own. CASE supplies IF-THEN logic to SQL statements and CAST can convert values from one datatype to another.

Date and Time

Performs operations on temporal datatypes and returns values in a temporal datatype format. There is no SQL2003 function that operates on a temporal datatype and returns a temporal result. The closest function is EXTRACT (covered in Section 4.4.3 later in this chapter), which operates on temporal values and returns numeric values. Functions returning temporal values but operating on no arguments are covered in the later Section 4.4.1.

 

4.5. Platform-Specific Extensions

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This section provides an alphabetical listing of DB2-supported functions that are specific to DB2 and not from the ANSI standard, with examples and corresponding results.

Synonym for ABS( number ). For example:

Returns the arc cosine of a number ranging from -1 to 1. The result ranges from 0 to and is expressed in radians. For example:

Returns the ASCII code of the first character of text. For example:

Returns the arc sine of number ranging from -1 to 1. The resulting value ranges from -/2 to /2 and is expressed in radians. For example:

Returns the arctangent of any number. The resulting value ranges from -/2 to /2 and is expressed in radians. For example:

Returns the arctangent of number and nbr. The values for number and nbr are not restricted, but the results range from - to and are expressed in radians. ATAN2( x,y ) is similar to ATAN( y/x ) with the exception that the signs of x and y are used to determine the quadrant of the result. For example:

 

5.1. Database Programming Overview

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Developing successful database applications, large and small, involves many steps. Careful thought must be given to application architecture, and especially to the following issues:

How to map application data, which is typically object-oriented, to a relational database

How to handle errors gracefully

How to maximize performance and scalability

A typical database application will require many different SQL statements. The management of so many statements is simplified by the fact that all SQL statements will follow roughly the same pattern of execution within an application. Figure 5-1 is a state diagram showing how SQL statements are prepared, executed, and then processed by a database application when interacting with a relational database system. The state diagram has been broken down into eleven steps, four of which are optional (and are indented in the diagram).

Figure5-1.Statement execution state diagram

Following are detailed descriptions of each step shown in Figure 5-1:

Establish connectivity : Establishing connectivity is the first step in every successful database application. It is in this step that the client, or database application, makes a physical connection to the database that will be used to transmit the SQL statements to the database and the results back to the client. The actual physical connection could be over a LAN, WAN, or even a simple logical connection back to the client in cases where the database application and server are running on the same machine. For more information on how to establish connectivity, please see the section Section 5.2 later in this chapter.

 

5.2. Opening a Database Connection

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Opening a connection with ADO.NET requires instantiating a connection object with a properly formatted connection string and then invoking the Open method on the connection object. The connection object can be an OdbcConnection, SqlConnection, or an OleDbConnection. The OdbcConnection is designed for any ODBC datasource, and the OleDbConnection type will work with any OLE DB Provider. For the highest performance data access, use connection objects specifically tuned to the specific database platform, such as SqlConnection for Microsoft SQL Server. Following is the syntax for creating a Connection object in ADO.NET:

The format of the connection string is the same for all of the connection types. The format is a string of key/value pairs delimited by semicolons. For example:

While the format is the same for every connection type, the keys and values are quite different. Tables Table 5-1 through Table 5-3 list the attributes for the three connection types listed above. Many database platforms support additional attributes that can also be set through the connection string. For a list of these attributes, please consult the appropriate database vendor documentation.

 

5.3. Closing a Database Connection

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