barcode generator in vb.net code project Creating and Altering Tables in Software

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Creating and Altering Tables
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As you might imagine, the process of deleting a table and its stored data is very straightforward. The following syntax shows you how easy this process is:
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DROP TABLE <table name>{ CASCADE | RESTRICT }
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The only real decision you need to make when deleting the table is whether to choose the CASCADE option or the RESTRICT option. As in previous syntax examples, the two options determine whether you should delete the table and its data if the table is being referenced by other objects. If CASCADE is used, the table and its data are deleted, along with any views, constraints, routines, or triggers that reference the table. If RESTRICT is used, the table is deleted only if no such dependencies exist. (As with the DROP COLUMN clause, SQL Server does not support CASCADE or RESTRICT, and Oracle permits only CASCADE CONTRAINTS.) For example, the following statement deletes the ARTISTS table and the data stored in the column, regardless of dependencies:
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DROP TABLE ARTISTS CASCADE;
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What if you want to delete the data in a table, but not the table definition itself Rather than using the DROP TABLE statement, you would use the DELETE statement. The DELETE statement deletes all rows from a table or deletes only specific rows, as defined within the statement. This is not the same as the DROP TABLE statement, which removes the table definition and the data. I discuss the DELETE statement in more detail in 8. Many product implementations also provide a TRUNCATE statement that provides a quick and efficient way to clear all the data out of a table. However, the TRUNCATE statement is not included in the SQL:2006 standard. You state that when a default value is defined for a column, the value is automatically inserted into the column when you add a row to the table but don t specify a value for that particular column. What happens if your column definition doesn t include a default and you try to insert that row The action taken depends on whether null values are permitted within the column. A null value means that the value is not known. This is not the same as a zero, blank, or default. If a null value is present, then the data is not available. By default, all columns permit null values, although you can override the default (discussed in 4). If you try to insert a row without specifying a specific value, a null value will be inserted into that column if the column permits null values. If the column does not permit null values, you will not be able to insert a row without defining a specific value for that column.
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SQL: A Beginner s Guide
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I ve often heard the term indexes discussed in relation to creating SQL tables. How do you create indexes Oddly enough, the SQL:2006 standard does not support the creation and maintenance of indexes, nor does it provide a definition or mention them in any other way. For those of you not familiar with them, an index is a set of search values and pointers (in a subsidiary table) that correspond to rows in a table. Indexes speed up queries and improve performance, making data access much more efficient, much like using the index of a book helps you find things more quickly than sequentially searching the pages. As a result, nearly every RDBMS supports some form of indexing, and indeed they are an important part of that product. However, the method used to implement indexing varies greatly, so each product provides its own system to set up and maintain their indexes. For example, the CREATE INDEX statement is available in most products; however, the syntax for the statement can vary considerably. As always, be sure to review the product documentation.
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