Pivot Query in Oracle

I recently revamped some stored procedures I wrote before and found a case where I can apply pivot query to simplify the code. Though I have seen the technique demonstrated in Tom Kyte' Expert One-on-One Oracle(ch.12), this is just another evidence that to know is one thing, to know where to apply is another.

The code I maintained read buy and sell trades from trade table
and sum them into position table for each symbol. Originally, I sumed
buy orders first and then sell orders and stored the results into position table. By using DECODE function, I can finish the task in fewer lines of code. Here is how.

Assuming my trade table and pos table are as follows.
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I want to sum records in trade table and store the summations in pos table. Using Pivot query as the sql in red square, I can get it done this way.
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This is what I want.
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