Saturday, July 23, 2011

Update Statement


Once there's data in the table, we might find that there is a need to modify the data. To do so, we can use the UPDATE command. The syntax for this is
UPDATE "table_name"
SET "column_1" = [new value]
WHERE {condition}

For example, say we currently have a table as below:
Table Store_Information
store_nameSalesDate
Los Angeles$1500Jan-05-1999
San Diego$250Jan-07-1999
Los Angeles$300Jan-08-1999
Boston$700Jan-08-1999

and we notice that the sales for Los Angeles on 01/08/1999 is actually $500 instead of $300, and that particular entry needs to be updated. To do so, we use the following SQL query:
UPDATE Store_Information
SET Sales = 500
WHERE store_name = "Los Angeles"
AND Date = "Jan-08-1999"

The resulting table would look like
Table Store_Information
store_nameSalesDate
Los Angeles$1500Jan-05-1999
San Diego$250Jan-07-1999
Los Angeles$500Jan-08-1999
Boston$700Jan-08-1999

In this case, there is only one row that satisfies the condition in the WHERE clause. If there are multiple rows that satisfy the condition, all of them will be modified. If no WHERE clause is specified, all rows will be modified.
It is also possible to UPDATE multiple columns at the same time. The syntax in this case would look like the following:
UPDATE "table_name"
SET column_1 = [value1], column_2 = [value2]
WHERE {condition}

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