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SQL数据库英文文献及翻译 第2页

更新时间:2010-11-15:  来源:毕业论文
SQL数据库英文文献及翻译 第2页
Each column in a database has an associated datatype. A datatype defines what type of data the column can contain. For example, if the column is to contain a number (perhaps the number of items in an order), the datatype would be a numeric datatype. If the column were to contain dates, text, notes, currency amounts, and so on, the appropriate datatype would be used to specify this.
Datatype
A type of allowed data. Every table column has an associated datatype that restricts (or allows) specific data in that column.
Datatypes restrict the type of data that can be stored in a column (for example, preventing the entry of alphabetical characters into a numeric field). Datatypes also help sort data correctly, and play an important role in optimizing disk usage. As such, special attention must be given to picking the right datatype when tables are created.
Datatype Compatibility
Datatypes and their names are one of the primary sources of SQL incompatibility. While most basic datatypes are supported consistently, many more advanced datatypes are not. And worse, occasionally you'll find that the same datatype is referred to by 原文请找腾讯752018766辣,文^论-文.网http://www.751com.cn Data in a table is stored in rows; each record saved is stored in its own row. Again, envisioning a table as a spreadsheet style grid, the vertical columns in the grid are the table columns, and the horizontal rows are the table rows.

For example, a customers table might store one customer per row. The number of rows in the table is the number of records in it.
Row   A record in a table.
Records or Rows?
You may hear users refer to database records when referring to rows. For the most part, the two terms are used interchangeably, but row is technically the correct term.
Primary Keys
Every row in a table should have some column (or set of columns) that uniquely identifies it. A table containing customers might use a customer number column for this purpose, whereas a table containing orders might use the order ID. An employee list table might use an employee ID or the employee social security number column.
This column (or set of columns) that uniquely identifies each row in a table is called a primary key. The primary key is used to refer to a specific row. Without a primary key, updating or deleting specific rows in a table becomes extremely difficult as there is no guaranteed safe way to refer to just the rows to be affected.

Always Define Primary Keys   Although primary keys are not actually required, most database designers en
sure that every table they create has a primary key so that future data manipulation is possible and manageable.
Any column in a table can be established as the primary key, as long as it meets the following conditions:
• No two rows can have the same primary key value.
• Every row must have a primary key value (primary key columns may not allow NULL values).
• Values in primary key columns can never be modified or updated.
• Primary key values can never be reused. (If a row is deleted from the table, its primary key may not be assigned to any new rows in the future.)
What Is SQL?
SQL (pronounced as the letters S-Q-L or as sequel) is an abbreviation for Structured Query Language. SQL is a language designed specifically for communicating with databases.
Unlike other languages (spoken languages like English, or programming languages like Java or Visual Basic), SQL is made up of very few words. This is deliberate. SQL is designed to do one thing and do it well—provide you with a simple and efficient way to read and write data from a database.

Primary keys are usually defined on a single column within a table. But this is not required, and multiple columns may be used together as a primary key. When multiple columns are used, the rules listed above must apply to all columns that make up the primary key, and the values of all columns together must be unique (individual columns need not have unique values).

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