mysql - SQL Where Date is greater than X -

i trying run query;

select po_purchaseorderdetail.itemcodedesc,           po_purchaseorderdetail.quantityordered,         po_purchaseorderdetail.quantityreceived,          po_purchaseorderdetail.unitcost,         po_purchaseorderdetail.jt158_wtsalesorderno,         po_purchaseorderdetail.purchaseorderno,         po_purchaseorderheader.purchasename,         po_purchaseorderheader.purchaseorderdate po_purchaseorderdetail po_purchaseorderdetail, po_purchaseorderheader po_purchaseorderheader (po_purchaseorderdetail.purchaseorderno=po_purchaseorderheader.purchaseorderno)  , ***(po_purchaseorderheader.purchaseorderdate > '2013-12-31')*** order po_purchaseorderdetail.jt158_wtsalesorderno, po_purchaseorderdetail.purchaseorderno 

my problem is, date stored in yyyy-mm-dd format. need show data after date. so, because 2013-12-31 string rather number, operator > doesn't work. ideas?

try cast string date in following:

p.s. if purchaseorderdate not date datatype cast it: cast(po_purchaseorderheader.purchaseorderdate date)

select po_purchaseorderdetail.itemcodedesc,        po_purchaseorderdetail.quantityordered,        po_purchaseorderdetail.quantityreceived,         po_purchaseorderdetail.unitcost,         po_purchaseorderdetail.jt158_wtsalesorderno,         po_purchaseorderdetail.purchaseorderno,         po_purchaseorderheader.purchasename,         po_purchaseorderheader.purchaseorderdate   po_purchaseorderdetail po_purchaseorderdetail,         po_purchaseorderheader po_purchaseorderheader  po_purchaseorderdetail.purchaseorderno = po_purchaseorderheader.purchaseorderno ,            po_purchaseorderheader.purchaseorderdate > cast('2013-12-31' date) order po_purchaseorderdetail.jt158_wtsalesorderno,           po_purchaseorderdetail.purchaseorderno