TAILIEUCHUNG - SQL Clearly Explained- P5

SQL Clearly Explained- P5: You don’t need to be a database designer to use SQL successfully. However, you do need to know a bit about how relational databases are structured and how to manipulate those structures. | 200 Chapter 8 Data Modification The result is 27 rows copied into the summary table one for each unique ISBN in the volume table. Note Should you store summary data like that placed in the table created in the preceding example The answer is it depends. If it takes a long time to generate the summary data and you use the data frequently then storing it probably makes sense. But if you can generate the summary data easily and quickly then it is just as easy not to store it and to create the data whenever it is needed for output. Placement of New Rows Where do new rows go when you add them That depends on your DBMS. Typically a DBMS maintains unique internal identifiers for each row that is not accessible to users something akin to the combination of a row number and a table identifier to provide information about the row s physical storage location. These identifiers continue to increase in value. If you were to use the SELECT syntax on a table you would see the rows in internal identifier order. At the beginning of a table s life this order corresponds to the order in which rows were added to the table. New rows appear to go at the bottom of the table after all existing rows. As rows are deleted from the table there will be gaps in the sequence of row identifiers. However the DBMS does not reuse them to fill in the holes until it has used up all available identifiers. If a database is very old very large and or very active the DBMS will run out of new identifier and will then start to reuse those made available by deleted rows. In that case new rows may appear anywhere in the table. Give that you can view rows in any order by using the ORDER BY clause it should make absolutely no difference to an end user or an application program where a new row is added. Please purchase PDF Split-Merge on to remove this watermark. Updating Data 201 Although most of today s end users modify existing data using an on-screen form the SQL statements to modify the data

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