Aggregate Functions
SQL defines aggregate functions to simplify server-side aggregate calculations on data. Except for the COUNT(*) function, all aggregate functions perform a Null-elimination step, so that Null values are not included in the final result of the calculation.
Note that the elimination of Null values is not equivalent to replacing those values with zero. For example, in the following table, AVG(i) (the average of the values of i) will give a different result from that of AVG(j):
| i | j |
|---|---|
| 150 | 150 |
| 200 | 200 |
| 250 | 250 |
| NULL | 0 |
Here AVG(i) is 200 (the average of 150, 200, and 250), while AVG(j) is 150 (the average of 150, 200, 250, and 0). A well-known side effect of this is that in SQL AVG(z) is not equivalent with SUM(z)/COUNT(*).
Read more about this topic: Null (SQL)
Famous quotes containing the words aggregate and/or functions:
“The aggregate of all knowledge has not yet become culture in us. Rather it would seem as if, with the progressive scientific penetration and dissection of reality, the foundations of our thinking grow ever more precarious and unstable.”
—Johan Huizinga (1872–1945)
“One of the most highly valued functions of used parents these days is to be the villains of their children’s lives, the people the child blames for any shortcomings or disappointments. But if your identity comes from your parents’ failings, then you remain forever a member of the child generation, stuck and unable to move on to an adulthood in which you identify yourself in terms of what you do, not what has been done to you.”
—Frank Pittman (20th century)