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not specify a maximum number of columns in a table, but almost all commercial SQL products do impose a limit. Usually the limit is 255 columns per table or more. Unlike the columns, the rows in a table do not have any particular order. In fact, if you use two consecutive database queries to display the contents of a table, there is no guarantee that the rows will be listed in the same order twice. Of course you can ask SQL to sort the rows before displaying them, but the sorted order has nothing to do with the actual arrangement of the rows within the table. A table can have any number of rows. A table of zero rows is perfectly legal and is called an empty table (for obvious reasons). An empty table still has a structure, imposed by its columns; it simply contains no data. The ANSI/ISO standard does not limit the number of rows in a table, and many SQL products will allow a table to grow until it exhausts the available disk space on the computer. Other SQL products impose a maximum limit, but it is always a very generous one two billion rows or more is common.
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Because the rows of a relational table are unordered, you cannot select a specific row by its position in the table. There is no first row, last row, or thirteenth row of a table. How, then, can you specify a particular row, such as the row for the Denver sales office In a well-designed relational database, every table has some column or combination of columns whose values uniquely identify each row in the table. This column (or columns) is called the primary key of the table. Look once again at the OFFICES table in Figure 4-7. At first glance, either the OFFICE column or the CITY column could serve as a primary key for the table. But if the company expands and opens two sales offices in the same city, the CITY column could no longer serve as the primary key. In practice, ID numbers, such as an office number (OFFICE in the OFFICES table), an employee number (EMPL_NUM in the SALESREPS table), and customer numbers (CUST_NUM in the CUSTOMERS table), are often chosen as primary keys. In the case of the ORDERS table, there is no choice the only thing that uniquely identifies an order is its order number (ORDER_NUM). The PRODUCTS table, part of which is shown in Figure 4-8, is an example of a table where the primary key must be a combination of columns. The MFR_ID column identifies the manufacturer of each product in the table, and the PRODUCT_ID column specifies the manufacturer s product number. The PRODUCT_ID column might make a good primary key, but there s nothing to prevent two different manufacturers from using the same number for their products. Therefore, a combination of the MFR_ID and PRODUCT_ID columns must be used as the primary key of the PRODUCTS table. Every product in the table is guaranteed to have a unique combination of data values in these two columns. The primary key has a different unique value for each row in a table, so no two rows of a table with a primary key are exact duplicates of one another. A table where every row is different from all other rows is called a relation in mathematical terms. The name relational database comes from this term, because relations (tables with distinct rows) are at the heart of a relational database.
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Figure 4-8.
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A table with a composite primary key
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Although primary keys are an essential part of the relational data model, early relational database management systems (System/R, DB2, Oracle, and others) did not provide explicit support for primary keys. Database designers usually ensured that all of the tables in their databases had a primary key, but the DBMS itself did not provide a way to identify the primary key of a table. DB2 Version 2, introduced in April 1988, was the first of IBM s commercial SQL products to support primary keys. The ANSI/ ISO standard was subsequently expanded to include a definition of primary key support, and today, most relational database management systems provide it.
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