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Dropping SQL Triggers
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Although the SQL standard does not support any sort of statement that allows you to alter a trigger, it does support a way to delete a trigger, which you achieve by using the DROP TRIGGER statement. As you can see in the following syntax, this statement is quite basic: DROP TRIGGER <name> All you need to do is provide the name of the trigger, along with the DROP TRIGGER keywords. Because no other objects are dependent on the trigger, you do not need to specify any additional keywords, such as CASCADE or RESTRICT. When you execute the DROP TRIGGER statement, the trigger definition is deleted from the schema.
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Create Insert Triggers
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So far in this chapter, I ve provided you with background information about triggers and the syntax used to create triggers. Now we ll look at examples of how triggers are created and what happens when they re invoked. We ll begin with the insert trigger, which, as you know, is invoked when an INSERT statement is executed against the subject table (the table on which the trigger has been defined). In the first example, we ll create a trigger on the RETAIL_ INVENTORY table (subject table), shown in Figure 14-2. The trigger, when invoked, will insert data into the INVENTORY_LOG table. The following CREATE TRIGGER statement defines an INSERT trigger that fires after the data is inserted into the subject table:
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CREATE TRIGGER INSERT_LOG AFTER INSERT ON RETAIL_INVENTORY FOR EACH ROW BEGIN ATOMIC INSERT INTO INVENTORY_LOG (ACTION_TYPE) VALUES ('INSERT'); END;
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RETAIL_INVENTORY CD_NAME: VARCHAR (60) Famous Blue Raincoat Blue Court and Spark Past Light Kojiki That Christmas Feeling R_PRICE: AMOUNT: NUMERIC (5,2) INT 16.99 14.99 14.99 15.99 15.99 10.99 5 10 12 11 4 8 14
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INVENTORY_LOG ACTION_TYPE: DATE_MODIFIED: CHAR (6) TIMESTAMP INSERT UPDATE UPDATE DELETE INSERT INSERT UPDATE UPDATE DELETE UPDATE 2002-12-22 10:58:05.120 2002-12-22 12:02:05.033 2002-12-22 16:15:22.930 2002-12-23 11:29:14.223 2002-12-23 13:32:45.547 2002-12-23 15:51:15.730 2002-12-23 17:01:32.270 2002-12-24 10:46:35.123 2002-12-24 12:19:13.843 2002-12-24 14:15:09.673
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Figure 14-2
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Creating an insert trigger on the RETAIL_INVENTORY table
14:
Creating SQL Triggers
NOTE
As I mentioned at the beginning of the chapter, SQL implementations can vary widely with regard to the semantics of the CREATE TRIGGER statement. For example, SQL Server does not allow you to specify a FOR EACH clause, nor does it support the use of the ATOMIC keyword in the BEGIN...END statement. On the other hand, the basic Oracle trigger definition is a lot closer to the SQL standard, although Oracle also does not support the use of the ATOMIC keyword in a trigger definition.
Let s take a look at this statement one element at a time. In the first line, the CREATE TRIGGER clause defines a trigger named INSERT_LOG. In the next line, the AFTER keyword is used to specify that the triggered SQL statements will be executed after the data has been inserted into the subject table. The AFTER keyword is followed by the INSERT keyword, which defines the trigger as an insert trigger. Next is the ON clause, which specifies the name of the subject table. In this case, the subject table is RETAIL_INVENTORY. As we move through the statement, we come to the FOR EACH clause, which specifies the ROW keyword. This clause, when used with ROW, indicates that the trigger will be invoked for each row that is inserted into the table, rather than for each INSERT statement that is executed against the table. Following the FOR EACH clause are the triggered SQL statements. The triggered SQL statements include a BEGIN...END statement and an INSERT statement. I did not need to include the BEGIN...END statement in the trigger definition because, without it, there is only one triggered SQL statement. However, I wanted to demonstrate how the block would be used had there been more than one statement. Notice that the block includes the ATOMIC keyword following the BEGIN keyword. According to the SQL standard, ATOMIC is required, although it will depend on your SQL implementation whether the keyword is supported. The BEGIN...END block encloses an INSERT statement that adds data to the INVENTORY_LOG table when the trigger is invoked. Each time a row is inserted into the RETAIL_INVENTORY table, a row is inserted into the INVENTORY_LOG table. The INVENTORY_LOG row will contain the INSERT value for the ACTION_TYPE column. A timestamp value is then added automatically to the DATE_MODIFIED column, which is defined with the default CURRENT_TIMESTAMP. You can, if you want, create other triggers on the RETAIL_INVENTORY table. For example, you might want to create update and delete triggers that insert rows into the INVENTORY_LOG table when the applicable data modifications are made. In that case, you would simply create a trigger definition for each additional trigger that you need.
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