In SQL, the CREATE AGGREGATE statement is used to define a user-defined aggregate function, allowing you to create custom aggregation behavior beyond what is built into your database system. The exact syntax and capabilities may vary based on the database management system (DBMS) you are using (such as PostgreSQL, SQL Server, or others). Here, I will provide an example using PostgreSQL, which supports the CREATE AGGREGATE statement.
The general steps to create a user-defined aggregate function in PostgreSQL are:
1. Create the State Data Type: Define a data type to hold the state of the aggregation process.
2. Create Functions: Define several functions that will be used by the aggregate: one to initialize the state, one to update the state with new input values, and one to finalize the result.
3. Create the Aggregate: Use the CREATE AGGREGATE statement to tie everything together.
Example Scenario: Creating a Custom Aggregate Function
We'll create a simple example aggregate function called my_avg, which computes the average of input numbers manually.
Step 1: Create the State Data Type
You can use an existing data type or create a composite type to hold your aggregate's state. In this case, we need to hold a sum and a count.
CREATE TYPE my_avg_state AS (
total_sum FLOAT,
total_count INTEGER
);
Step 2: Create Functions for the Aggregate
1. Initialize the State:
CREATE FUNCTION my_avg_state_init() RETURNS my_avg_state AS $$
BEGIN
RETURN ROW(0, 0)::my_avg_state; -- Initialize with sum = 0 and count = 0
END;
$$ LANGUAGE plpgsql;
2. State Transition Function: This function updates the state with new values.
CREATE FUNCTION my_avg_state_transition(state my_avg_state, value FLOAT) RETURNS my_avg_state AS $$
BEGIN
RETURN ROW(state.total_sum + value, state.total_count + 1)::my_avg_state;
END;
$$ LANGUAGE plpgsql;
3. Final Function: This function produces the final result.
CREATE FUNCTION my_avg_state_final(state my_avg_state) RETURNS FLOAT AS $$
BEGIN
IF state.total_count = 0 THEN
RETURN NULL; -- Handle division by zero if no values were aggregated
ELSE
RETURN state.total_sum / state.total_count; -- Compute average
END IF;
END;
$$ LANGUAGE plpgsql;
Step 3: Create the Aggregate
Now, you can define the aggregate using the functions you've created.
CREATE AGGREGATE my_avg (
SFUNC = my_avg_state_transition, -- State transition function
STYPE = my_avg_state, -- State type
INITCOND = '0,0', -- Initial condition
finalfunc = my_avg_state_final -- Final result function
);
Usage
Once defined, you can use your custom aggregate function in a SQL query like this:
SELECT my_avg(column_name) FROM your_table;
Summary
This is a general outline of how to create a user-defined aggregate function using the CREATE AGGREGATE statement in PostgreSQL. Be aware that the exact syntax and capabilities may differ if you are working with a different DBMS. Always check the database documentation for specifics regarding user-defined aggregate functions.