Beginning SQL

CREATE Table MyTable(FirstColumn int, SecondColumn int);
INSERT INTO MyTable(FirstColumn,SecondColumn)
VALUES (3,2);
INSERT INTO MyTable(FirstColumn)
VALUES (5);
INSERT INTO MyTable(SecondColumn)
VALUES (7);
Now try the following query:
SELECT FirstColumn, SecondColumn, 10 + (SecondColumn * 1.175) + FirstColumn - 5
FROM MyTable;
The query calculates SecondColumn * 1.175, then adds 10, and then adds the value of FirstColumn
minus 5. You get results that are mostly NULL because of NULL values in FirstColumn or SecondColumn:
FirstColumn SecondColumn 10 + (SecondColumn * 1.175) + FirstColumn - 5
3 2 10.350
5 NULL NULL
NULL 7 NULL
The last column in the first row of the table is calculated like this: 10 + (SecondColumn * 1.175) +
FirstColumn – 5 = 10 + (2 * 1.175) + 3 – 5 = 10 + (2.35) + 3 – 5 = 10.35.
The third column in the second row is calculated like this: 10 + (SecondColumn * 1.175) +
FirstColumn – 5 = 10 + (NULL * 1.175) + 5 – 5 = 10 + (NULL) + 5 – 5 = NULL.
The final answer is NULL because any time NULL data is involved the answer comes out NULL. If you
were asked to compute 1 plus 1, the answer is obviously 2. However, if you were asked to add 1 to some
unknown number, all you can say is that you don’t know — the answer is unknown. Remember, NULL
represents unknown data rather than any specific value.
It’s easy to forget that if NULL is involved at any point in a math calculation, the result always comes out
NULL, so you need to make sure the program using the data can handle NULLs. Otherwise, if the data is
displayed directly to a user, they may wonder what NULL means!
To circumvent these problems, you can choose either to filter out NULL results or to use one of the functions
(discussed shortly) to change NULL to something the end user can understand.
The same problem of unexpected NULL results applies if you use the math functions, such as POWER(),
ABS(), and so on. A NULL value causes the function to return NULL, which propagates to the whole
expression, leaving a final result of NULL.
MyTable is no longer needed and can be dropped from the database:
DROP TABLE MyTable;
Manipulating Data
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