Storing conditional logic expressions/rules in a database

Question

How can I store logical expressions using a RDBMS?

I tag objects and would like to be able to build truth statements based on those tags. (These might be considered as

Answer 1

I would use one table

tags(id,name,type,expression,order)

• type would show if the tag is normal or calculated.
• order is reordered if you add new calculated tags, it specifies the order of the calculation for these tags...
• expression is parsed and checked before inserting a row, it could also be built using a GUI (something like how Oracle discoverer does these things).
• You only link the normal tags to the items

For your example second-hand-goods needs to be calculated before second-hand-offer, all the others can be calculated without any dependencies.

1,'new',1,'',NULL
2,'for_sale',1,'',NULL
3,'used',1,'',NULL
4,'offer',1,'',NULL
5,'second_hand_goods',2,'(!new or used) and for_sale',1
6,'new_offer',2,'new and offer',1
7,'second_hand_offer',2,'second_hand_goods and offer',2

An item could be tagged by only for_sale, calculating would give:

second_hand_goods,second_hand_offer

I would have a function that gives a list of all the tags for the item, including direct tags and calculated ones:

for_sale,second_hand_goods,second_hand_offer