Tài liệu MASTERING SQL SERVER 2000- P9 pdf

W e’re going to go out on a limb here and assume that you own stuff— such as clothes, food, VCRs, tools, etc. Most people keep the stuff they own in their homes, but where? Do you just randomly throw your stuff in your house and hope you can find it again later? Of course not—you store your belongings in containers, such as cabinets or dressers, so that you can find your belongings when you need them. Now go one step further: Do you keep all of your stuff in the same container? Imagine the chaos that would ensue if you kept your tools, food, and clothes in the same cabinet—you would not be able to find anything when you needed it. These principles hold true with SQL Server. The stuff you own in SQL Server is things such as tables, views, stored procedures, and other objects. Much like with your clothes, food, tools, etc., you need containers to store those objects in—with SQL Server, those containers are databases. Again, go one step further: Do you want to keep all of your objects in the same database? Defi- nitely not. Just as when you store all of your personal belongings in the same cabinet, you would have a terrible time sorting out all of the data if it was all in one database. That is why you need to have more than one database, each dedicated to a specific task, such as an accounting database to hold all of the accounting objects and data, or a sales database for the sales objects and data. It makes sense, then, that before you start creating objects, such as tables and views, you must create the database that will contain those objects. That is what this chapter deals with: creating, configuring, and administrating databases. We’ll start by reviewing the basics of how a database works. Database Basics As with anything, you need to understand the basics before you can jump into the more advanced topics—this is especially true with databases. As we mentioned in Chapter 3, a database is a series of files on your hard disk. These files are just space that has been preallocated on the hard disk for storing other SQL Server objects, such as tables and views. These files on the hard disk can be one of three types: a primary data file, a secondary data file, and a transaction log file. The primary data file (with an .MDF extension) is the first file created for the data- base. This file can be used to store two types of objects: user and system objects. User objects are such things as tables, views, stored procedures, and the like that are used to modify or store information that has been input by a user. System tables contain infor- mation that SQL Server needs to keep your database functioning, such as table names, index locations, database user accounts, and information about other system objects. The system tables must reside in the primary data file, but the user information and other objects can be moved to secondary data files. 2627ch10.qxt 8/22/00 10:45 AM Page 370 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 371 When you run out of room on the hard disk that contains the primary data file, you can create a secondary data file (with an .NDF extension) on a separate hard disk. Once you have created the secondary file, you can use it to store user data, such as tables, indexes, and views, but not system objects (those reside only in the primary data file). The third type of file requires a little more explanation than the data files. The third type of file is the transaction log file, and it functions much like a constant online backup by storing transactions. A transaction is a group of data modification commands (for example, INSERT, UPDATE, and DELETE) that is contained in a BEGIN TRAN…COMMIT block and executed as a unit, meaning that all of the commands in the transaction are applied to the database, or none of them are. There are two types of transactions that SQL Server understands: implicit and explicit. An implicit transaction occurs when you send a data modification command to SQL Server without specifi- cally encasing it in a BEGIN TRAN…COMMIT block—SQL Server will add the block for you. An explicit transaction occurs when you specifically type the BEGIN TRAN and COMMIT statements at the beginning and end of your statement block. A typical explicit transaction might look as follows: BEGIN TRAN INSERT RECORD DELETE RECORD COMMIT TRAN SQL Server sees the INSERT and DELETE commands as a single unit of modifica- tion—either they both happen or neither happens, or in SQL Server terminology, they are either rolled forward or rolled back. The DELETE cannot happen without the INSERT and vice versa. Every command in SQL Server that modifies data is considered a transaction, each having a BEGIN and COMMIT statement, whether or not you put them there (if you don’t add the BEGIN and COMMIT, SQL Server will). You might expect each of these transactions to be written directly to the database file, but that is not the case. When a user tries to modify a record in a database, SQL Server locates the data page (pages are discussed in Chapter 3) in the database that contains the record to be changed. Once located, the page in question is loaded into memory—specifically, it is loaded into a special area of memory called the data cache, which SQL Server uses to store data that is to be modified. All of the changes to the page are now made in memory (or RAM, random access memory), because RAM is about 100 times faster than hard disk, and speed is of the essence. NOTE As discussed in Chapter 3, a page is 8KB and is the smallest unit of storage in a SQL Server database. DATABASE BASICS Digging into SQL Server PART III 2627ch10.qxt 8/22/00 10:45 AM Page 371 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. CHAPTER 10 • DATABASES 372 Leaving those changed records in RAM is a bad idea, though, because RAM is considered volatile, which means that all of the contents of RAM are erased every time the computer loses power. If the machine were to lose power, you would lose all of the changes in the data cache. So rather than leaving those changes at the mercy of RAM, SQL Server writes the changes made in the data cache to the transac- tion log at the same time. Now you have a copy of the data in RAM and on the hard disk in the transaction log file. If the server were to lose power now, all of the changes stored in the data cache would be erased, but you could still recover them from the transaction log. In that sense, the transaction log is like a constant online backup of the data cache. So why not just write all of the changes from data cache directly to the database file? Why put the transaction log in the middle? Imagine what would happen to your database if your server were to crash right in the middle of writing changes from memory to the data file if there were no transaction log. The transaction would be partially written to disk, and the original transaction would be erased from memory with no hope of recovery. However, because the transaction is written to the transac- tion log first, if the server crashes, the original transaction is preserved, and partial transactions are not written to the database. In fact, if a crash occurs, SQL Server reads the transaction logs for each database looking for completed transactions that have not been applied to the data file. If SQL Server finds any, it rolls them forward, writing them to the data file. Any uncom- pleted transactions (a BEGIN TRAN with no corresponding COMMIT) are rolled back or deleted from the transaction log. This way, you can recover your databases right up to the minute of a crash. Because of the benefits that you gain from transaction logs, they are required for each database—you cannot have a primary data file without a transaction log. The transaction log file (with an .LDF extension) should be placed on a separate physical hard disk than the data file. If the hard disk with the data file crashes, you still have the transaction log file and the last good backup to re-create the data file on a new hard disk. The transaction log file should be approximately 10 to 25% of the size of the data files to accommodate the transactions made during the day. If your users do not make many modifications to the data, you can go with a smaller transaction log (10% being the minimum), whereas if your users are constantly modifying the data, you should make the transaction log file larger (maybe even up to 30%). NOTE Because all of the changes are written to the transaction log before they are written to the data file, the transaction log is referred to as a write ahead log. 2627ch10.qxt 8/22/00 10:45 AM Page 372 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 373 Now that you know how these files work, you need to know how big to make them. Let’s look at capacity planning. Planning for Capacity Perhaps you’ve heard the old adage waste not, want not. That rings true regarding hard-disk space on your SQL Server. Because databases are files that are stored on your hard disk, you can actually waste hard-disk space if you make them too big. If you make your database files too small, though, SQL Server will have to expand the data- base file, or you may need to create a secondary data file to accommodate the extra data—a process that can slow users down. Neither of these options is very appealing, so you need to find a happy balance between too big and too small, which is going to require a little math. Here are the general steps to estimate the size of your database: 1. Calculate the record size of the table in question. You get this by adding the size of each column in the table. 2. Divide 8092 by the row size from step 1 and round down to the nearest number. The figure 8092 is the actual amount of data a single data page can hold, and you round down because a row cannot be split across pages. 3. Divide the number of rows you expect to have by the result from step 2. This will tell you how many data pages will be used for your table. 4. Multiply the result from step 3 by 8192—the size of a data page in bytes. This will tell you exactly how many bytes your table will take on the disk. In Chapter 11, you will learn how to plan a database—deciding what tables to put in it, what datatypes to use, and how big the fields in the tables should be—so we’ll forego that discussion here. In this section we’re going to assume that the planning phase is complete and create a sales database that will contain three tables: one for customer information, one for product information, and one for order detail informa- tion. To calculate the size of your new database, let’s apply the following steps to the customers table to discern how big it will be with 10,000 records: 1. Assuming you have already planned your database, add all of the field sizes in the customers table together. Here is the table layout (you should get 125 bytes): custid int (note: this is 4 bytes of storage) fname varchar(20) lname varchar(20) address varchar(50) PLANNING FOR CAPACITY Digging into SQL Server PART III 2627ch10.qxt 8/22/00 10:45 AM Page 373 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. CHAPTER 10 • DATABASES 374 city varchar(20) state char(2) zip char(9) 2. Divide 8092 by 125 and round down to the nearest number to find out how many of these rows can fit on a single data page. You must round down in every case because a row cannot span a page. The answer should be 64. 3. Divide 10,000 (the estimated number of rows in the table) by the number of rows on a page (64) and round up to the nearest number. You round up here because a partial row will be moved to a whole new page—there is no such thing as a partial page of storage. The answer should be 157. 4. Multiply 157 (the number of pages required to hold 10,000 records) by 8192 (the size of a page on disk). This should be 1,570,000 bytes. So, with 10,000 records, the customers table in your sales database would require approximately 1.5MB of hard-disk space. By repeating these steps for each table in the database, you can figure out approximately how much space to allocate to the data- base when you first create it. With all of the math out of the way, you are ready to start creating a database. Creating Databases We discussed earlier that a database is comprised of at least two files: first, the primary data file (with an .MDF extension) and the transaction log file (with an .LDF exten- sion). There may also be a need for secondary data files if the hard disk that contains the primary data file fills up, but we will discuss those later in this chapter. To get started with the database, you only need to create the primary data file and transaction log file. There are three different ways to go about it: • By using the Create Database Wizard • Graphically with Enterprise Manager • Via Transact-SQL code We’ll look at each method here, starting with the Create Database Wizard. TIP New databases are actually a copy of the Model database, because Model has all of the system objects necessary for any database to function. This means that if you want any standard objects in all of your databases (for example, a database user account), if you add the object to the Model database, the object will automatically exist in all new databases. 2627ch10.qxt 8/22/00 10:45 AM Page 374 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 375 Using the Create Database Wizard Wizards, if you are not familiar with them, are a series of step-by-step screens that help you accomplish a task with which you may not be familiar. Although Wizards are most useful for the novice, they can also be a great help to the seasoned adminis- trator. Wizards not only provide you with a step-by-step process for accomplishing a task, they also perform all of the menial work involved, allowing you to focus on the more advanced tasks that come later. The Create Database Wizard is no exception; we will use it here to create a simple trial database, just to get the feel of the Wizard: 1. If you are not in Enterprise Manager, open it now by selecting it from the SQL Server 2000 group in Programs on the Start menu. 2. On the Tools menu, select Wizards. 3. Expand Database and select Create Database Wizard. Click OK to start the Wizard. 4. The opening screen displays a list of what this Wizard is designed to accom- plish. Click Next to proceed. 5. On the second screen, you are asked for a name for the database and the loca- tion of the data and log files. For the name, enter Wizard Test and leave the defaults for the file locations. Click Next. CREATING DATABASES Digging into SQL Server PART III 2627ch10.qxt 8/22/00 10:45 AM Page 375 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. CHAPTER 10 • DATABASES 376 6. The third screen prompts you for the size of the data file; enter 5 to make the file 5MB, then click Next. 7. The next screen gives you the option to have the database file automatically expand when more space is required for data. Leave the defaults here and click Next; we’ll discuss file growth shortly. 2627ch10.qxt 8/22/00 10:45 AM Page 376 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 377 8. You are asked for the size of the transaction log file. Remembering that this should be about 10 to 25% of the size of the data file, you will leave the default of 1MB and click Next. 9. You are asked if you would like the transaction log to automatically expand. Click Next to accept the defaults. CREATING DATABASES Digging into SQL Server PART III 2627ch10.qxt 8/22/00 10:45 AM Page 377 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. CHAPTER 10 • DATABASES 378 10. The final screen gives a list of the options that you have chosen. Verify that these are what you want and click Finish to create your database. 11. When asked if you would like to create a maintenance plan for the database, click No. You will learn how to create a maintenance plan in Chapter 17. 2627ch10.qxt 8/22/00 10:45 AM Page 378 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 379 12. To verify that the Wizard Test database exists, expand Databases under your server and click Wizard Test (if it exists). You should see an information screen pop up in the contents pane (on the right). You may need to refresh the tree- view in the left pane by right-clicking your server and selecting Refresh to see the new database. Using the Create Database Wizard is probably the simplest way to create a data- base, but because there are eight screens to deal with, this method takes a little longer than the next method, using Enterprise Manager. Creating Databases with Enterprise Manager The next easiest way to create a database in SQL Server is through Enterprise Manager. This method does not detail each step of database creation and is therefore considered to be a slightly more advanced method than using the Wizard. Using Enterprise Man- ager to create a database is also a little faster than using the Wizard because there are only three screens with which to deal. To help you get the feel of using Enterprise Man- ager for creating databases, we will use this next series of steps to create a sales database that can later be filled with tables, views, and other objects for a sales department: 1. Open Enterprise Manager from the SQL Server 2000 group in Programs on the Start menu and expand your server; then expand the Databases icon. CREATING DATABASES Digging into SQL Server PART III 2627ch10.qxt 8/22/00 10:45 AM Page 379 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [...]... something new to SQL Server 2000— the listbox at the bottom of the screen labeled Compatibility Level This is designed to force your database to behave like one in an earlier version of SQL Server This is useful for older applications that have not yet been updated to function with SQL Server 2000 You will notice three settings here: 60, 65, and 70 The 60 and 65 settings will cause the SQL Server database... smallest unit of storage in SQL Server is an 8KB page, but when SQL Server writes a page to hard disk, the page is written 512 bytes at a time because hard disks store information in 512-byte sectors If a power failure occurs while SQL is writing a page to disk, you may get only part of that page on disk, which is called a torn page When Torn Page Detection is checked, SQL Server marks each 512-byte... the database statistics Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark 8/22/00 10:45 AM Page 383 CREATING DATABASES 383 TI P When you create a new object in SQL Server, you may not see it in the contents (right) pane right away Right-clicking the level just above where your new object should be and selecting Refresh will force SQL Server to reread the system tables and... duplicates) If this option is checked, SQL Server will automatically create statistics for any column that is part of an index If this option is unchecked, you must create your own statistics Again, it is best to leave this turned on until you understand SQL Server well enough to outsmart the query optimizer Auto Update Statistics: Setting this option will instruct SQL Server to automatically update your... (note that you should replace the C:\ with the drive on which you installed SQL Server) : CREATE DATABASE DoomedDB ON PRIMARY (name = DoomedDB, filename = ‘c:\Program Files\Microsoft SQL Server\ data\DoomedDB.mdf’, size = 10MB, maxsize = 15MB, filegrowth = 1MB) LOG ON (name = DoomedLog, filename = ‘c:\Program Files\Microsoft SQL Server\ data\DoomedLog.ldf’, size = 2MB, maxsize = 3MB, filegrowth = 10%) 3... ready to start filling the databases with objects In the next chapter, let’s start by creating tables Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark PA R T Digging into SQL Server III This page intentionally left blank Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark 2627ch11.qxt 8/23/00 10:26 AM Page 405 CHAPTER 11 Tables F E AT U R I N G : Planning... The only time to have this option off is if you have a disk cache with a battery backup that is specially designed for database servers; otherwise leave this option checked Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark PA R T III Digging into SQL Server 2627ch10.qxt 2627ch10.qxt 390 8/22/00 10:45 AM Page 390 CHAPTER 10 • DATABASES Auto Close: When a user connects to a database,... space on your hard disk To verify that this database has been created, open Enterprise Manager and expand your server and then databases Notice DoomedDB in the list of available databases Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark PA R T III Digging into SQL Server 2627ch10.qxt 2627ch10.qxt 386 8/22/00 10:45 AM Page 386 CHAPTER 10 • DATABASES Now that your database is... which they are members Dbcreator is Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark Digging into SQL Server III 2627ch10.qxt 388 8/22/00 10:45 AM Page 388 CHAPTER 10 • DATABASES another special group with privileges inside a database Sysadmin is a special group that has administrative control over every database on the server When this option is checked, only members of... purchase PDF Split-Merge on www.verypdf.com to remove this watermark 8/22/00 10:45 AM Page 397 MODIFYING DATABASES 397 3 On the Transaction Log tab, on the second line of the Transaction Log Files section, type doomed_log2 in the File Name field and notice that the rest of the fields are filled in for you Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark PA R T III Digging into SQL . storage in a SQL Server database. DATABASE BASICS Digging into SQL Server PART III 2627ch10.qxt 8/22/00 10:45 AM Page 371 Please purchase PDF Split-Merge. the SQL Server 2000 group in Programs on the Start menu and expand your server; then expand the Databases icon. CREATING DATABASES Digging into SQL Server PART III 2627ch10.qxt