Storing conditions for further selecting from a database. Design challenge - database

Question

Let’s assume I have a online computer shop and I want to add a new feature - advertising.
As a manager of the shop I want to create an advert about particular product. Let’s say it goes like this: “IF a client is already signed up for at least 2 months AND during purchase the value of his order exceeded 150$ OR product A OR B is already in his shopping cart THEN show him this ad”.
My question is how to store such statements (condition “A or B”, “A and B”, “(A or B) and C”, etc.) in a database and then, how to select the records and display (or not) desired ad?
One of my idea:
Adverts
1. id,
2. name,
3. description,
…
4. criterias_pattern [i.e “(1 OR 5) AND 4”]
Second table:
AdvertsCriterias
1. id
...
2. type
3. value
In short:
I parse the pattern stored in “criterias_pattern” field, extract criterias_id and then I check the conditions.
It should work but it has many obvious drawbacks.

Answer 1

random thought -
why not store actual sql WHERE clauses that would do the evaluation. at least then you do not need to reinvent your own syntax and parsers.

Answer 2

You have two problems: Modeling the conditions, and storing them in a DB.
Modeling the conditions: The easier would probably be if you could use an existing scripting language model, or something like that. If you can't you can model the conditions like:
interface Expression
{
public T Evaluate(Context context);
}
class OrExpression : Expression
{
Expressionleft;
Expressionright;
}
class AndExpression : Expression
{
Expressionleft;
Expressionright;
}
class IfExpression : Expression
{
Expressioncondition;
ExpressionthenClause;
ExpressionelseClause;
}
class EqualExpression : Expression
{
Expression left;
Expression right;
}
class ContextVariableValue : Expression
{
}
“IF a client is already signed up for at least 2 months AND during purchase the value of his order exceeded 150$ OR product A OR B is already in his shopping cart THEN show him this ad”
would be something like that:
var clientIsAlreadySignedUpFor2Months = new GreaterThanExpression(new ContextVariableValue("ClientSignedUpLengthInMonths"), new ConstantExpression(2));
var purchaseExceeds150 = new GreaterThanExpression(new ContextVariableValue("PurchaseAmount"),new ConstantExpression(150));
new IfExpression(new ) )
var result = new AndExpression(clientIsAlreadySignedUpFor2Months, purchaseExceeds150, etc...)
To store it in the DB you can either serialize them to some equation like text and then parse them, or you could use something like
Conditions
Id Type ParametersType Operand1 Operand2
1 GreaterThanExpression Number 2 3
2 ContextVariableValue Number ClientSignedUpLengthInMonths null
3 ConstantExpression Number 2 null
etc...
I think your best bet, is to preload all the advertisments with their conditions in memory, and run them every time you could be displaying an add...
If you detect that a lot of adds have "similar" conditions, then you could add some logic to only evaluate those conditions once, to speed up things, etc...