Conditional SQL Query

I want to write sql statement about product, product_tree_special,
product_tree_template

product has
product_code varchar(20) PRIMARY KEY,
product_name varchar(20)

product_tree_template has
product_code varchar(20) PK,
stock_code_to_make_product(20) PK,
amout float4

product_tree_spcial has
production_no serial,
product_code varchar(20) PK,
stock_code_to_make_product(20) PK,
amout float4

NOW : product_tree_template table is the default table for production
ingredients of the PRODUCT

but sometimes my customer using special product_tree table for some
production_no

how can I write a query to get right ingredients of a product basis on
production_no field

such as;

first check production_no if product_tree_special table if not found then
look at template table...

What are your suggestions?

PS: I do not wanna write PL functions for this...

Thanks

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