Ben beat me to it - looking at the sql in the jrxml files is very useful. What you also want is a test database and the mysql workbench - though the sql editor in Jaspersoft's Studio product is almost as good - though its syntax checker is a bit wonky.

here is the parameter input screen for one of my reports:

Here is the sql that implements the above:

select
    product.name as product,
    customer.name as customer,
    customer.entity_id as customer_id,
    patient.name as patient,
    patient.entity_id as patient_id,
    child.activity_start_time as date,
    if(credit,-f.quantity, f.quantity) as quantity,
    if(credit,-f.total,f.total)as total
from acts parent join act_relationships r on parent.act_id = r.source_id
    join acts child on r.target_id = child.act_id and child.arch_short_name like "act.customerAccount%Item"
    join participations product_partic on child.act_id = product_partic.act_id
    join entities product on product.entity_id = product_partic.entity_id and product.arch_short_name like "product.%"
    left join participations patient_partic on child.act_id = patient_partic.act_id and patient_partic.arch_short_name = "participation.patient"
    left join entities patient on patient.entity_id = patient_partic.entity_id and patient.arch_short_name like "party.patient%"
    join participations cust_partic on parent.act_id = cust_partic.act_id and cust_partic.arch_short_name = "participation.customer"
    join entities customer on customer.entity_id = cust_partic.entity_id and customer.arch_short_name like "party.%"
    join financial_acts f on f.financial_act_id = child.act_id
    left join participations pl ON parent.`act_id` = pl.`act_id` and pl.arch_short_name = "participation.location"
    left join `entities` l ON pl.`entity_id` = l.`entity_id`
    left join entity_classifications productgroup on productgroup.entity_id = product.entity_id
    left join lookups grouplookup on grouplookup.lookup_id = productgroup.lookup_id
    left join entity_relationships rpt on rpt.target_id = product.entity_id and rpt.arch_short_name = "entityRelationship.productTypeProduct"
    left join entities producttype on producttype.entity_id = rpt.source_id
where parent.activity_start_time between $P{startDate} and date_add($P{endDate}, INTERVAL "23:59:59" HOUR_SECOND)
    and product.name like concat(ifnull($P{productName},""),"%")
    and customer.name like concat(ifnull($P{customerName},""),"%")
    and l.name like concat(ifnull($P{locationName},""),"%")
    and if($P{classification} is null,(grouplookup.name like concat(ifnull($P{classification},""),"%") or grouplookup.lookup_id is null),grouplookup.name like concat(ifnull($P{classification},""),"%"))
    and if($P{productType} is null,(producttype.name like concat(ifnull($P{productType},""),"%") or producttype.entity_id is null),producttype.name like concat(ifnull($P{productType},""),"%"))

group by child.act_id
order by customer.name, patient.name, child.activity_start_time

As Ben said, you do need to look at the archetypes - ie fire up your openvpms test system and use Administration|Archetypes.  Also read http://www.openvpms.org/documentation/csh/1.8/admin/archetype#archetypeDef

Also, if you are trying to understand OpenVPMS, look at http://www.openvpms.org/documentation/csh/1.8/concepts

A couple of other comments:

1. in OpenVPMS there is no such thing as a deleted invoice or deleted invoice line item. You can delete these while the invoice is being generated (and they disappear from the database) - but once it is finalised you cannot
2. tax inc/ex - see http://www.openvpms.org/documentation/csh/1.8/concepts/taxes [note that I wrote the above report for a practice in Hong Kong were there are no sales or VAT or GST taxes hence there is no need to show tax in/ex amounts - they are the same.
3. Product 'description' - we call this the name - but there are both Printed Names and Names - if there is a Printed Name this will be displayed on the invoice rather that  the Name. Reports (like the one above) generally show the Name.
4. unit price - be careful - products can also have a fixed price - see http://www.openvpms.org/documentation/csh/1.8/concepts/pricing

Have fun, regards Tim G