Tài liệu Oracle SQL Jumpstart with Examples- P3 docx

70 3.5 Enhancing the Physical Architecture safety can be transferred to a standby as they are created using the Log Writer (LGWR), filling a precreated archive log file on the standby data- base. On the other hand, maximum performance can be achieved at the expense of safety, thus potentially presenting possible data loss using the Archiver (ARCn) to transfer log entries from primary to standby. In this case, redo log entries are transferred when a primary database log switch occurs, copying each archive log file to a standby database as it is created. Using the Archiver, redo log entries are not copied as they are created but only after primary database archiving. Physical standby has disadvantages. A physical standby can only be accessed externally in read-only mode, and it must duplicate the source (primary) database exactly. A logical standby database is maintained in read-write mode, a completely open and accessible database. Also, a logical standby can have a subset of source database objects and can even contain objects in addition to the primary database. Once again, logical standby is much more flexible than physical standby. 3.5.5 Clustering and Oracle RAC Clustering was previously called Oracle Parallel Server and is now called Oracle Real Application Clusters (RAC). Oracle RAC allows for sharing of a single large data source’s data across more than one Oracle instance, run- ning on more than a single database server. Thus multiple database servers share the same data, allowing for high availability, enormous scalability, and flexibility. Figure 3.12 Oracle Standby/ Fail-over Database Architecture. Chap3.fm Page 70 Thursday, July 29, 2004 10:00 PM Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 3.5 Enhancing the Physical Architecture 71 Chapter 3 So far, this book has examined the underlying logical and physical struc- ture of Oracle Database plus new features available in both Oracle Database 10g and Oracle Database 9i. Now it’s time to begin looking into Oracle SQL itself. The next chapter begins this process by examining the SELECT statement. Chap3.fm Page 71 Thursday, July 29, 2004 10:00 PM Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. This page intentionally left blank Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 73 4 The SELECT Statement In this chapter:  How do you write a basic query using SELECT statements?  What types of SELECT statements are possible?  What else is interesting about SELECT statements? In this chapter, we dive right into the syntax and use of the SELECT statement to query the database. We also briefly discuss different types of queries, finally examining specific aspects of queries such as using DIS- TINCT and the DUAL table. So let’s begin with the basics of the SELECT statement and some simple examples just to get into the swing of things. 4.1 The Basic SELECT Statement SELECT is the beginning of the SQL command for querying (retrieving) data from a database table, view, or object. Objects are similar to tables, but they have a more complex structure. 4.1.1 Uses of the SELECT Statement The SELECT statement is a specialized way to ask a question about the data in a database. Thus a SELECT statement is also called a query because it quite literally “queries” or asks questions of a database. There are several uses for the SELECT statement that give you great flexibility in the database:  Simple query . A SELECT statement can be used alone to retrieve data from a table or a group of related tables. You can retrieve all col- Chap4.fm Page 73 Thursday, July 29, 2004 10:03 PM Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 74 4.1 The Basic SELECT Statement umns or specify some columns. You can retrieve all rows or specify which rows you want.  Complex query . A SELECT statement can be embedded within another SELECT statement. This lets you write a query within a query. The possibilities are endless. Later chapters cover the details.  Create a view or table . A SELECT statement can be used to create a view or a new table. A view is a stored query that is executed when- ever another SELECT statement retrieves data from the view by using the view in a query. Views are very useful to enforce security by limiting the columns or rows that particular users are allowed to see.  Insert, update, or delete data . A SELECT statement can be used within the INSERT, UPDATE, or DELETE statements to add greater flexibility to these commands. Chapter 15 examines com- mands for manipulating data. Note: There are numerous other more detailed types of queries using the SELECT statement to be described briefly later in this chapter and in detail in later chapters. 4.1.2 Syntax Conventions In this section, and throughout the rest of the book, you will see SQL and SQL*Plus commands listed first with their syntax and then with many examples, some of which you could type yourself to help you better under- stand the commands. The syntax of a command defines the set of rules governing the correct form of a command. Some parts are required and never change, others are optional, and others vary with each different statement. Figure 4.1 shows the syntax of the basic SELECT statement with descriptions of the vari- ous parts. Here is the basic syntax of the SELECT statement in a textual form (Backus-Naur Form), as shown in Figure 4.1. See Chapter 1 for details of Backus-Naur syntax formatting. SELECT { [alias.]column | expression | [alias.]* [ , … ] } FROM [schema.]table [alias]; Chap4.fm Page 74 Thursday, July 29, 2004 10:03 PM Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 4.1 The Basic SELECT Statement 75 Chapter 4 Curly braces mean you must choose from one of the choices between them. So, you can either write a list of column names, or write an expres- sion, or use *. An asterisk (*) represents all column names within a query when used in the SELECT statement. Square brackets mean you can include the items within the square brack- ets or leave them out entirely. In the SELECT command, you can list just one column or many columns, or even simply an asterisk (*) if you choose. The lowercase words are always replaced with actual names of tables, columns, schemas, and so on. The words in the syntax give you a hint on what should be used. This structure is just the bare bones of the SELECT command. Other chapters cover all of the many variations and options available for the SELECT command. The complete syntax definition of the SELECT command in Oracle’s SQL documentation takes up five pages. The description of all the variables in the command takes up another 25 pages. In this book, you will build gradually on your knowledge, chapter by chapter, until you have enough knowledge of the SELECT command to write complex queries easily. Note: Details of Backus-Naur syntax conventions can be found in Chapter 1. This book almost always follows a slight variation on that theme, described in Chapter 1. Any variations are generally specific to particular chapters and noted at the beginning of those chapters. Let’s look at a few examples. Figure 4.1 The Syntax of the SELECT Statement. Chap4.fm Page 75 Thursday, July 29, 2004 10:03 PM Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 76 4.1 The Basic SELECT Statement 4.1.3 Some Simple Example SELECT Statements The first example retrieves rows from an Oracle metadata view: SELECT VIEW_NAME, TEXT FROM USER_VIEWS; This statement has a list of two columns (VIEW_NAME and TEXT), and the view queried is named USER_VIEWS. Tables and views are inter- changeable in the SELECT command. No schema name is used because the view in this case belongs to the user who is running the query. As a gen- eral rule, any time you query a table or view that belongs to the user you log in as, no schema name is required. Likewise, when you query a table or view that is in another user’s schema, you must use the schema name. For example, if you log in as JOE and you want to query a table name CARS owned by SAM, you would have to add the schema name CARS. SELECT * FROM SAM.CARS; Note: The semicolon is technically not considered part of the SQL state- ment’s syntax. The semicolon marks the end of the statement and submis- sion. A forward slash on a blank line following the SQL statement serves the same purpose. Submission means submission to the SQL engine, in other words “execute it!” Now let’s do some simple SELECT statement examples using the MUSIC schema. Note: Diagrams and scripts for the MUSIC schema are in Chapter 1 and Appendix A. Let’s begin with a query listing all the data in the MUSICCD table: SELECT * FROM MUSICCD; Figure 4.2 shows the result. Notice the blank spaces in certain columns. This stands for a null value in the data. For example, the PLAYING_TIME column for the first row (Soak Up the Sun) is NULL. Chap4.fm Page 76 Thursday, July 29, 2004 10:03 PM Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 4.1 The Basic SELECT Statement 77 Chapter 4 To select specific columns, the asterisk could be changed to something like PRESSED_DATE, TITLE, MUSICCD_ID, listing columns in the sequence specified. SELECT PRESSED_DATE, TITLE, MUSICCD_ID FROM MUSICCD; The next query contains a calculation between two columns. You can add, subtract, multiply, divide, and use parentheses to affect the calculation order of factors in expressions. When you combine columns, include calcu- lations, or other operations, an expression is created. Expressions can be used in a SELECT statement anywhere you use a column. SELECT ARTIST_ID, SESSION_DATE, AMOUNT_CHARGED-AMOUNT_PAID FROM STUDIOTIME; Observe that the column heading of the third column is AMOUNT_CHARGED - AMOUNT_PAID. This is long, and if you Figure 4.2 SQL*Plus Report Layout. Chap4.fm Page 77 Thursday, July 29, 2004 10:03 PM Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 78 4.1 The Basic SELECT Statement were handing a report off to someone else, you might want a more descrip- tive heading. To change the heading, add a column alias to the SELECT statement. A column alias redefines a column’s heading in a SELECT state- ment. In this example, we change the second line by adding the alias “Bal- ance Due.” AMOUNT_CHARGED-AMOUNT_PAID "Balance Due" Using double quotes preserves the upper and lowercase appearance of the heading. Without double quotes, your alias will always appear in upper- case letters in the report. Additionally, in this case because the words “Bal- ance” and “Due” are separated by a space, “Due” will be interpreted as a column name, causing an error. Figure 4.3 shows the output. Now add aliases to all three columns and change the SELECT statement again: SELECT ARTIST_ID Artist , SESSION_DATE "In Studio" , AMOUNT_CHARGED-AMOUNT_PAID " Balance Due " FROM STUDIOTIME; Figure 4.3 Column Aliases Can Help Make Queries More Readable. Chap4.fm Page 78 Thursday, July 29, 2004 10:03 PM Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 4.1 The Basic SELECT Statement 79 Chapter 4 Figure 4.4 shows the result. Headings have changed. Because the ARTIST_ID alias Artist is not in double quotes, the heading is displayed as uppercase even though it was typed in mixed case. Now add an alias to the table name. Although this action does not affect your report, it will be useful in the future when you create more complex queries. A table alias is a shortcut name that is used as a substitute for the table name in the SELECT statement. The table alias is best being short and simple, but it does not have to be. Note: I was once hired for a contract because I used single characters and not table names for table aliases. Why? Using table names to reference col- umns can make quite a mess of SQL statements. Using single-character aliases makes for much more readable, ultimately debuggable and tunable SQL code. The table alias should be added to all of the table’s columns (not column aliases) in the SELECT statement. This is a good habit to adopt because you will be able to create more readable SQL when using table aliases. Many of the examples in this book use table aliases. In this case, the letter S is used for the table alias: Figure 4.4 Three Column Aliases, with and without Double Quotes. Chap4.fm Page 79 Thursday, July 29, 2004 10:03 PM Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [...]... results retrieved with a query? Filtering eliminates rows from a query and is done with the WHERE clause Figure 4.6 shows all rows with artists containing the vowel “a” in their names SELECT ARTIST_ID, NAME FROM ARTIST WHERE NAME LIKE '%a%'; Note: The percentage character (%) is used as a wild card character representing zero or more characters Oracle SQL wild card characters used with the LIKE clause... SELECT statements are important to remember: The DUAL table is a dummy or temporary table used to execute non -SQL- type commands with the SQL command interpreter Using functions allows use of a large amount of built-in (provided) functionality or even custom-written functions Arithmetic is allowed in SQL using standard arithmetic operators The DISTINCT function allows retrieval of unique values from a row... NAME FROM ARTIST WHERE ARTIST_ID = ANY (SELECT GUESTARTIST_ID FROM GUESTAPPEARANCE); Those are the conditional comparisons available in Oracle SQL Now let’s look at what I like to call logical operators 5.3 Logical Operators in the WHERE Clause Logical operators in Oracle SQL are AND, OR, and NOT They work to concatenate multiple conditional expressions together Precedence rules apply in that expressions... different queries together Flashback or versions queries allow access to data at a previous point in time Parallel queries execute SQL statements in parallel, preferably using multiple CPU platforms and Oracle Partitioning Let’s look at some of the query types briefly, starting with the simple query Chapter 4 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark 82 4.2 Types of SELECT... Parallel queries are special queries designed to run faster in parallel and are best executed on dual-CPU platforms, particularly with Oracle Partitioning 4.3.1 The DUAL Table All DML statements create implicit cursors Cursors are memory chunks allocated for results of SQL statements SELECT statements require a source for an implicit cursor to operate on Some types of SELECT statements do not retrieve... always returns a null value The NVL (value, replace) function replaces null values in expressions, avoiding SQL errors The SET NULL environment variable does the same thing in SQL* Plus Null values sort as the highest value by default 4.3.6 Using Pseudocolumns One or two simple examples will suffice with respect to the SELECT statement For example, the following query finds the ROWID (logical row pointer)... query will find all artists with a letter “a” anywhere in their names The next example will only find artists with a letter “a” in the second position of their names SELECT * FROM ARTIST WHERE NAME LIKE '_a%'; Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark 5.2 WHERE Clause Expression Conditions 103 IN set membership evaluates an expression as being within a set of elements... these two statements are different: SELECT * from artist where name like ('%C%'); SELECT * from artist where name like ('%c%'); Oracle Database 10g ignores line breaks and spacing in SQL commands For example, the following two SELECT statements are identical when submitted in SQL* Plus, even though spacing and line breaks make them look completely different from each other SELECT Name , Street Please... numbers (the AMOUNT_CHARGED and the AMOUNT_PAID) Subtracting two dates (the SYSDATE and the DUE_DATE) When you subtract dates, Oracle Database 10g will calculate the number of days between the two dates Dates are stored internally as Julian dates and automatically converted with the default data display A Julian date is a number in seconds from a specified date, such as January 1, Chapter 4 Please purchase... schema Figure 4.11 shows a small section of this hierarchy This query will read a small section of the hierarchy including and contained within the Guitar node as shown in Figure 4.11 The result is shown in Figure 4.12 SELECT LEVEL, SECTION_ID, NAME FROM INSTRUMENT START WITH NAME = 'Guitar' CONNECT BY PRIOR INSTRUMENT_ID = SECTION_ID; Figure 4.11 The MUSIC Schema Instruments Hierarchy Please purchase . ture of Oracle Database plus new features available in both Oracle Database 10g and Oracle Database 9i. Now it’s time to begin looking into Oracle SQL itself Clustering and Oracle RAC Clustering was previously called Oracle Parallel Server and is now called Oracle Real Application Clusters (RAC). Oracle RAC allows