IF/Then statements

[nikkiheave]

Can someone tell me why this won't execute:

select ci.order_item_id "Order Item ID", ci.conc_spec_id, ci.num_substances, ci.testset_id, ci.reservation_id,
ci.bill_of_material_id, co.bom_type, cs.container_set_id from cos_order_item ci, cos_order_item_cont_set cs,
cos_order co where ci.order_item_id = cs.order_item_id and
co.order_id=ci.order_id
if co.bom_type = well
then select ci.bill_of_material_id from cos_order_item_cont_set
else co.bom_type = "null"
end if;

[Shestakov]

select ci.order_item_id "Order Item ID", ci.conc_spec_id, ci.num_substances, ci.testset_id, ci.reservation_id,
ci.bill_of_material_id, co.bom_type, cs.container_set_id from cos_order_item ci, cos_order_item_cont_set cs,
cos_order co where ci.order_item_id = cs.order_item_id and
co.order_id=ci.order_id;
------- ; ----- after select ---------
if co.bom_type = well then
select ci.bill_of_material_id from cos_order_item_cont_set;
else
co.bom_type := "null";
end if;

------- := ----- not = ---in co.bom_type := "null";------