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out in the original INSERT command, such as the customer s first name It is not uncommon to see this erroneous SQL code:
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test2=# update store"Customer" set "FirstName" = 'Barbara'; UPDATE 2 test2=#
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Your first clue that something bad happened would be the output of the UPDATE command The value after the update message shows the number of rows affected by the update process Since this value is 2, you know that more than just the one row you wanted to change has been changed Obviously something went wrong Doing a simple query on the data shows what went wrong:
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test2=# select * from store"Customer"; CustomerID | LastName | FirstName | Address | City |State| Zip | Phone + + + + + + + BLU001 | Blum | Barbara |123 Main St|Chicago| IL | 60633|555-1234 BLU002 | Blum | Barbara | | | | |555-4321 (2 rows) test2=#
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Ouch We managed to change the FirstName column in both records in the table This is an all-to-common mistake made by even the most experienced database programmers and administrators when in a hurry Without the WHERE clause portion of the UPDATE command, the update is applied to every record in the table In almost all cases, this is not what you intend to do The WHERE clause allows you to restrict the records that the UPDATE command applies to The WHERE clause is a logical statement that is evaluated by PostgreSQL Only records that match the condition contained within the WHERE clause are updated Here is an example of a simple WHERE clause:
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test2=# update store"Customer" set "FirstName" = 'Rich' test2-# WHERE "CustomerID" = 'BLU001'; UPDATE 1 test2=# select * from store"Customer"; CustomerID | LastName | FirstName | Address | City |State| Zip | Phone + + + + + + + BLU002 | Blum | Barbara | | | | |555-4321 BLU001 | Blum | Rich |123 Main St|Chicago| IL |60633|555-1234 (2 rows) test2=#
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Every record in the table is examined to determine if it meets the condition defined in the WHERE clause In this simplistic example, there is only one record that could possibly match the condition, as the primary key column cannot be duplicated in the table Only the record that met the WHERE clause condition was updated with the new information
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You can add as many elements within the condition to the WHERE clause as you need to restrict the records to a specific subset within the table, such as updating employee records for everyone making more than $100,000 You can also update more than one column in a single UPDATE command:
test2=# update store"Customer" set "Address" = '123 Main St', test2-# "City" = 'Chicago', test2-# "State" = 'IL', "Zip" = '60633' test2-# where "CustomerID" = 'BLU002'; UPDATE 1 test2=#
The WHERE option is where things can start to get complex You can specify expressions for just about any column from any table in the database to determine which columns are updated To use columns from other tables, you must precede the WHERE option with the FROM option, listing the tables the foreign columns come from:
test=# update store"Order" set "Quantity" = 5 from store"Customer" test-# where "Order""CustomerID" = "Customer""CustomerID"; UPDATE 1 test=#
Deleting Data
The last data function discussed is removing data that is no longer needed in the table Not surprisingly, the DELETE command is used to do this The format for the DELETE command is
DELETE FROM table [WHERE condition]
This command is similar to the UPDATE command Any records matching the condition listed in the WHERE clause are deleted As with the UPDATE command, extreme caution is recommended when using the DELETE command Another all-too-common mistake is to quickly type the DELETE command and forget the WHERE clause That results in an empty table, as all records are deleted Here are a couple of examples of using the DELETE command:
test2=# delete from store"Customer" where "CustomerID" = 'BLU001'; DELETE 1 test2=# delete from store"Customer" where "CustomerID" = 'BLU003'; DELETE 0 test2=#
Notice that when the DELETE command fails to find any records to delete it does not produce an error message Instead, it just reports back that the total number of records deleted was zero
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