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1、细心整理Introduction to SQL ServerBy SamuelRelational databases have been around for 30 years, but they were not the original kind ofdatabase, nor are they the newest kind of database. XML and object-oriented data structures haveevolved in recent years. But relational databases are still by far the most
2、 popular kind of database available and will be for some time to come.SQL is the abbreviation of Structured Query Language and it is for relational databases, as the title indicates this is only for fresher who has just started the carrier or who is waiting to open up the carrier in the application
3、programming side. But that does not mean this article is a tutorial for a fresher who does not know anything about SQL.This article is meant for who already have a little knowledge in SQL and want toimprove it.What Does SQL Do?First, SQL is the premier tool for viewing information from a relational
4、database. It doesnt just give you a data dump. SQL gives you sophisticated tools to summarize, consolidate, and calculate from the data. Using table relationships, data can be combined from multiple tables in a number of ways. With a properly designed database, SQL can answer practically any questio
5、n about the data.Second, SQL provides commands to manipulate the data in a relational database. Records can be updated and added to or deleted from a table. Here is SQL as a database language really shines. Procedural programming languages, such as BASIC, might require several lines of code to updat
6、e a record in a database table. In addition, procedural programming languages would have to use some sort of looping structure to repeat this process on every record. SQL operates on an entire set of records all at the same time. SQL is like haiku for programmers; often a dozen words or fewer can de
7、lete or change thousands of records.Finally, SQL is a complete data definition language (DDL). The database itself can be created along with all tables, fields, primary keys, and relationships. Add to that the record insert commands, and you can have a complete database and all its data expressed in
8、 programming code. This greatly enhances a database programmers ability to work remotely or to port data enhancements among various installations.The prerequisite for learning SQL is knowledge in Discrete Mathematics (Set Theory,Relations and Functions). Although it is not necessary to learn all the
9、 theorems and proof for the theorems in the Discrete Mathematics, you should have learned the basic concepts of the Sets, Relations and Functions. This will help you to learn SQL queries and fundamentals easily. If you want to explore a RDBMS more deeply you should learn Graph Theory too.Although I
10、tried to avoid SQL Server specific topics in this article, I am sure that some topics are pure to SQL server such as SQL Enterprise manager.Representation of Data by DBMSDBMS should represent the data stored by them in some form and it is most common to represent them as Column, Row, Tables and Data
11、bases. As you know SQL you know what are columns, rows, tables and database.Primary KeyA column or set of columns used to uniquely identify the records in a table. Primary Keys dont allow NULL values. You can relate a table with other table if you have defined primary on it. (So defining UNIQUE and
12、NOT NULL constraint is not equivalent to primary key). SQL server will create a Clustered index while you create a primary key.Primary keys should be carefully defined since based on it other relations are defined. Poorly designed primary keys will affect insert, update and delete operations. Primar
13、y keys are different from which are used for paperwork.Foreign KeyThe primary key of one of the tables is almost always involved in the relationship. The field in the other table on the other end of that relationship is called the foreign key. The termsimply refers to the fact that this field is key
14、 to relating to a foreign (or other) table. In the Lyric Music database there is a relationship between artists and titles. The ArtistID field is the primary key in the Artists table. Therefore, the ArtistID field in the Titles table is a foreign key. It relates the Titles table to the primary key i
15、n the Artists table.Most table relationships can be described as one-to-many. In a one-to-many relationship, a single record in the first table can be related to many records in the second table. However, each record in the second table relates to only one record in the first table. In addition to o
16、ne-to-many relationship, tables can have one-to-one relationships. But these are much less common.DDL (Data Definition Language)What is data type? Classification of data into similar groups. Names, Money,Date, Time, etc are examples for data type.What is the use of classification or data type? It increases the performance, reduces the space needed to store the data.DML (Data Manipulation Language)Data Manipulation Languages contains command
