I have a table that stores dob (mm/dd/yyyy)in MySQL DB. But now I need to aggregate that table's contents like this: 
Group 1: <= 16 years
Group 2: >=17 E <= 25
Group 3: >=26 E <= 40
Group 4: >=41 years
MySQL gives the ages, but how can I aggregate them in those groups?

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Ideally you would want to use the IF() function to get the aggregates for this table. The IF() function takes three arguments and returns a single value: 

mysql> SELECT IF(1 = 2, 'True', 'False');
+----------------------------+
| IF(1 = 2, 'True', 'False') |
+----------------------------+
| False                      |
+----------------------------+
1 row in set (0.44 sec)
As you can see, the first argument is evaluated, and if the first argument evaluates true, the second argument is passed, otherwise the third argument is passed. So how do we use this to aggregate the situation you provided? First we create the aggregates: 
IF(age <= 16, 1,  0)
IF(age >= 17 AND dob <=  25, 1, 0)
IF(age >=  26 AND dob <= 40, 1, 0)
IF(age >= 41, 1, 0)
Then we simply provide a sum of the columns and format it as a SELECT query: 
SELECT SUM(IF(age <= 16, 1,  0)) AS VeryYoung,
              SUM(IF(age >= 17 AND age <=  25, 1, 0)) AS Young,
              SUM(IF(age >=  26 AND age <= 40, 1, 0)) AS GettingThere,
              SUM(IF(age >= 41, 1, 0)) AS Others
FROM mytable;
Here's a sample of the output you can expect on a table with ages 5, 10, 15...45, 50:
+-----------+-------+--------------+--------+
| VeryYoung | Young | GettingThere | Others |
+-----------+-------+--------------+--------+
|         3 |     2 |            3 |      2 |
+-----------+-------+--------------+--------+
1 row in set (0.06 sec)
And there you have it. As you indicated, you already can determine the age, so we won't go into that in this question.

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This was first published in June 2004

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