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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