create aggregate

To create an aggregate in a database, you need to use the SQL command CREATE AGGREGATE.

The syntax for creating an aggregate is:

CREATE AGGREGATE aggregate_name (input_data_type) (
    sfunc = aggregate_function,
    stype = state_data_type,
    finalfunc = final_function,
    initcond = initial_condition
);

Here's what each component of the syntax means:

- `aggregate_name`: The name you want to give to the aggregate.
- `input_data_type`: The data type of the input values for the aggregate.
- `sfunc`: The function used to perform the aggregate calculation on each input value.
- `stype`: The data type of the state variable used to store intermediate results during the aggregate calculation.
- `finalfunc`: The function used to combine all of the intermediate results into a final result.
- `initcond`: The initial value of the state variable.

For example, to create an aggregate that calculates the average of a set of numbers, you could use the following SQL command:

CREATE AGGREGATE avg (numeric) (
    sfunc = sum,
    stype = numeric,
    finalfunc = avg_final,
    initcond = '0'
);

This creates an aggregate named `avg` that takes numeric input values. The `sum` function is used to calculate the sum of all input values, and the `numeric` data type is used to store the intermediate results. The `avg_final` function is used to calculate the final average, and the initial value of the state variable is set to `0`.