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Advanced SQL Database Programmer phần 6

Bây giờ, bạn có thể nghĩ rằng nếu chúng ta để điều trị FALSE và UNKNOWN như nhau, sau đó tại sao đi đến tất cả các rắc rối để xác định một logic ba có giá trị ở nơi đầu tiên? SQL có ba ngôn ngữ: DML, DDL, và DCL. Control Data Language (DCL) điều khiển người dùng truy cập vào cơ sở dữ liệu và không sử dụng các vị. Trong Ngôn ngữ thao tác dữ liệu (DML), người dùng có thể yêu cầu | Next create a table with one column and make it an IDENTITY column. Now try to insert update and delete different numbers from it. If you cannot insert update and delete rows from a table then it is not a table by definition. Finally create a simple table with one IDENTITY column and a few other columns. Use a few statements like INSERT INTO Foobar a b INSERT INTO Foobar a b INSERT INTO Foobar a b c VALUES al bl c1 c VALUES a2 b2 c2 c VALUES a3 b3 c3 to put a few rows into the table and notice that the IDENTITY column sequentially numbered them in the order in which they were presented. If you delete a row the gap in the sequence is not filled in and the sequence continues from the highest number that has ever been used in that column in that particular table. But now use a statement with a query expression in it like this INSERT INTO Foobar a b c SELECT x y z FROM Floob Since a query result is a table and a table is a set that has no ordering what should the IDENTITY numbers be The entire whole completed set is presented to Foobar all at once not a row at a time. There are n ways to number n rows so which one do you pick The answer has been to use whatever the physical order of the result set happened to be that nonrelational phrase physical order again. But it is actually worse than that. If the same query is executed again but with new statistics or after an index has been dropped or added the new execution plan could bring the result set back in a different physical order. DBAzine.com BMC.com oracle 51 Oh why did duplicate rows in the second query get different IDENTITY numbers In the relational model they should be treated the same if all the values of all the attributes are identical. There are better ways of creating identifiers but that is the subject for another column. In the meantime stop writing bad code until I can teach you how to write good code. 52 DBAzine.com BMC.com oracle Keyword Search Queries CHAPTER 9 Keyword Searches Here is a short problem .