javascript barcode scanner Figure 8-1 Examples of using the UPDATE statement in Software

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Figure 8-1 Examples of using the UPDATE statement
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In this example, the subquery (lines 3 and 4) will select every employee who is in a department whose name includes the string IT and increment their current salary by 10 percent (unlikely to happen in practice) It is also possible to use subqueries to determine the value to which a column will be set, as in the fourth example In this case, one employee (identified by primary key, in line 5) is transferred to department 80 (the sales department), and then the subquery in lines 3 and 4 sets his commission rate to whatever the lowest commission rate in the department happens to be The syntax of an update that uses subqueries is as follows:
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UPDATE table SET column=[subquery] [,column=subquery] WHERE column = (subquery) [AND column=subquery] ;
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There is a rigid restriction on the subqueries using update columns in the SET clause: the subquery must return a scalar value A scalar value is a single value of whatever data type is needed: the query must return one row, with one column If the query returns several values, the UPDATE will fail Consider these two examples:
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update employees set salary=(select salary from employees where employee_id=206); update employees set salary=(select salary from employees where last_name='Abel');
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The first example, using an equality predicate on the primary key, will always succeed Even if the subquery does not retrieve a row (as would be the case if there were no employee with EMPLOYEE_ID equal to 206), the query will still return a scalar value: a null In that case, all the rows in EMPLOYEES would have their SALARY set to NULL which might not be desired but is not an error as far as SQL is concerned The second example uses an equality predicate on the LAST_NAME, which is not guaranteed to be unique The statement will succeed if there is only one employee with that name, but if there were more than one it would fail with the error ORA-01427: single-row subquery returns more than one row For code that will work reliably, no matter what the state of the data, it is vital to ensure that the subqueries used for setting column values are scalar TIP A common fix for making sure that queries are scalar is to use MAX or MIN This version of the statement will always succeed: update employees set salary=(select max(salary) from employees where last_name='Abel'); However, just because it will work, doesn t necessarily mean that it does what is wanted The subqueries in the WHERE clause must also be scalar, if it is using the equality predicate (as in the preceding examples) or the greater/less than predicates If it is using the IN predicate, then the query can return multiple rows, as in this example which uses IN:
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update employees set salary=10000 where department_id in (select department_id from departments where department_name like '%IT%');
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This will apply the update to all employees in a department whose name includes the string IT There are several of these But even though the query can return several rows, it must still return only one column PART II EXAM TIP The subqueries used to SET column values must be scalar subqueries The subqueries used to select the rows must also be scalar, unless they use the IN predicate Exercise 8-2: Use the UPDATE Command In this exercise, use various techniques to update rows in a table It is assumed that the WEBSTOREPRODUCTS table is as seen in the illustration at the end of Exercise 8-1 If not, adjust the values as necessary 1 Connect to the WEBSTORE schema using SQL Developer or SQL*Plus 2 Update a single row, identified by primary key:
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update products set product_description='DBA1 Exam Guide' where product_id=102;
This statement should return the message 1 row updated 3 Update a set of rows, using a subquery to select the rows and to provide values:
update products set product_id=(1+(select max(product_id) from products where product_id <> 102)) where product_id=102;
This statement should return the message 1 row updated 4 Confirm the state of the rows:
select * from products;
5 Commit the changes made:
commit;
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