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ueries are the heart of SQL, and many people use SQL as a database query tool. The next five chapters describe SQL queries in depth. 5 describes the basic SQL structures that you use to form SQL statements. 6 discusses simple queries that draw data from a single table of data. 7 expands the discussion to multitable queries. Queries that summarize data are described in 8. Finally, 9 explains the SQL subquery capability that is used to handle complex queries.
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CHAPTER 5 SQL Basics CHAPTER 6 Simple Queries CHAPTER 7 Multitable Queries (Joins) CHAPTER 8 Summary Queries CHAPTER 9 Subqueries and Query Expressions
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his chapter begins a detailed description of the features of SQL. It describes the basic structure of a SQL statement and the basic elements of the language, such as keywords, data types, and expressions. How SQL handles missing data through NULL values is also described. Although these are basic features of SQL, they have some subtle differences in the way they are implemented by various popular SQL products, and in many cases, the SQL products provide significant extensions to the capabilities specified in the ANSI/ISO SQL standard. These differences and extensions are also described in this chapter.
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The main body of SQL consists of about 40 statements. The most important and frequently used statements are summarized in Table 5-1. (Note that not all SQL implementations support all these statements.) Each statement requests a specific action from the DBMS, such as creating a new table, retrieving data, or inserting new data into the database. All SQL statements have the same basic form, illustrated in Figure 5-1. Every SQL statement begins with a verb, a keyword that describes what the statement does. CREATE, INSERT, DELETE, and COMMIT are typical verbs. The statement continues with one or more clauses. A clause may specify the data to be acted on by the statement or provide more detail about what the statement is supposed to do. Every clause also begins with a keyword, such as WHERE, FROM, INTO, and HAVING. Some clauses are optional; others are required. The specific structure and content vary from one clause to another. Many clauses contain table or column names; some may contain additional keywords, constants, or expressions. The ANSI/ISO SQL standard specifies a set of reserved keywords and nonreserved keywords that are used within SQL statements. According to the standard, reserved keywords cannot be used as the exact name of database objects, such as tables, columns, and users. Many SQL implementations relax this restriction, but it s generally a good idea
Part II:
Retrieving Data
Statement Data Manipulation SELECT INSERT UPDATE MERGE DELETE Data Definition CREATE TABLE DROP TABLE ALTER TABLE CREATE VIEW DROP VIEW CREATE INDEX DROP INDEX CREATE SCHEMA DROP SCHEMA CREATE DOMAIN ALTER DOMAIN DROP DOMAIN Access Control GRANT REVOKE CREATE ROLE GRANT ROLE DROP ROLE Transaction Control COMMIT ROLLBACK SET TRANSACTION START TRANSACTION SAVEPOINT TABLE 5-1
Description Retrieves data from the database Adds new rows of data to the database Modifies existing database data Conditionally inserts/updates/deletes new and existing rows Removes rows of data from the database Adds a new table to the database Removes a table from the database Changes the structure of an existing table Adds a new view to the database Removes a view from the database Builds an index for a column Removes the index for a column Adds a new schema to the database Removes a schema from the database Adds a new data value domain Changes a domain definition Removes a domain from the database Grants user access privileges Removes user access privileges Adds a new role to the database Grants role containing user access privileges Removes a role from the database Ends the current transaction Aborts the current transaction Defines data access characteristics of the current transaction Explicitly starts a new transaction Establishes a recovery point for a transaction
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