The Language of SQL- P38 pdf

Although the examples in the present chapter were focused on data retrieval, stored procedures and functions are also quite useful in applying data updates.. chapter 17Modifying Data VAL

Looking Ahead

In this chapter, we’ve seen that the use of parameters can add a great deal of

flexibility to the process of retrieving data For example, parameters allow you to

generalize SQL statements so that values for selection criteria can be specified at

the time the statement is executed You’ve also learned the basics of how to create

and modify stored procedures Finally, we explained some of the differences

between stored procedures and user-defined functions

Although the examples in the present chapter were focused on data retrieval,

stored procedures and functions are also quite useful in applying data updates

Our next chapter, ‘‘Modifying Data,’’ will take us completely out of the realm of

data retrieval and move us squarely into issues surrounding the need to update

data The business of maintaining data doesn’t present the same analytic

possi-bilities as data retrieval, but it is a necessary task for any enterprise Fortunately,

most of the techniques you’ve learned with theSELECTstatement apply equally

to the modification processes to be discussed in the next chapter

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chapter 17

Modifying Data

VALUES , DELETE , TRUNCATE TABLE , UPDATE

Having exhausted our discussion of retrieving data from databases, we will now move on to the topic of how to modify data There are three basic scenarios as to how data can be modified:

■ Inserting new rows into a table

■ Deleting rows from a table

■ Updating existing data in specific rows and columns in a table

As you may surmise, inserting and deleting rows is relatively straightforward Updating existing data, however, is a more complex endeavor We’ll begin with the insert and delete functions and then conclude with updates

Modification Strategies

The mechanics of modifying data are fairly straightforward However, the nature

of the process means that this is an area fraught with danger Being human, mistakes can be made With a single command, you can easily delete thousands

of rows in error Or you can apply numerous updates that may be difficult to retract

As a practical matter, there are various strategies that can be employed to help prevent catastrophic blunders For example, when deleting rows from a table,

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you can employ a soft delete technique, which means that instead of actually

deleting rows, you can denote a special column in a table that marks each row as either active or inactive Rather than deleting a row, you merely mark a row as being inactive That way, if a delete is done in error, you can easily reverse it by changing the value of the active/inactive column

A similar technique can be utilized when doing inserts When adding a row, you can mark in a special column the exact date and time of the insert If it is later determined that the row was added in error, you can find all rows added in a specified time range and have them deleted

The problem is more complex when it comes to updating data Generally, it’s advisable to maintain a separate table that holds data on intended update trans-actions If any kind of error is made, you can go back to the transaction table to look up the before-and-after values for data that was modified and use that to reverse any earlier mistakes

The above mentioned strategies are just a few of the many approaches that can be taken This is a topic that is well beyond the scope of this book Just be sure that you are very cautious when updating data Unlike many user-friendly desktop applications, there is no undo command in SQL

Inserting Data

SQL provides anINSERTkeyword to add data into a table There are two basic ways in which anINSERTcan be done:

■ Insert specific data that is listed in anINSERTstatement

■ Insert data that is obtained from aSELECTstatement

Let’s start with an example that shows how to insert data, where the data values are specified in theINSERT statement Let’s assume that we have a Customers table with this data already in it:

CustomerID FirstName LastName State

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Let’s also assume that the first column, CustomerID, is the primary key for the

table Back in Chapters 1 and 2, we talked about the fact that primary keys

enforce the requirement that each row in a table should be uniquely identifiable

We also mentioned that it is often the case that primary key columns are

speci-fied as auto-increment columns This means that they are automatically assigned

a number as a row is added to the table

Assuming that the CustomerID column is defined as an auto-increment column,

this means that when we add a row to the Customers table, we don’t specify

a value for the CustomerID column It will be automatically determined as a

row is added to the table We only need to specify values for the other three

columns

Let’s proceed with an attempt to add two new customers to the table: Virginia

Jones from Ohio and Clark Woodland from California

This statement performs the insert:

INSERT INTO Customers

(FirstName, LastName, State)

VALUES

('Virginia', 'Jones', 'OH'),

('Clark', 'Woodland', 'CA')

After the insert, the table contains:

CustomerID FirstName LastName State

A few words of explanation are in order First, notice that theVALUESkeyword

is used as a prefix to lists of values to be inserted into the table The statement

lists each row of data in a separate set of parentheses Virginia Jones of Ohio was

in one set of parentheses; Clark Woodland was in another set The two sets were

separated by a comma If we needed to only add one row, then just one set of

parentheses would be needed

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