Temporary columns - problem z warunkiem Opcje

[ekstro]

Cześć, mam taki problem:

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SELECT t1.id, t1.op_name,
 
@invoice_sum_tax:=(SELECT sum((b1.price_base*b1.quantity)*(b3.rate+1)) FROM t24_invoices_elements AS b1 LEFT JOIN t24_taxes AS b3 ON b3.id = b1.code_taxes WHERE code_invoices = t1.id ) AS invoice_sum_tax, 
 
@payments_sum:=(SELECT sum(c1.value) FROM t24_payments_list AS c1 WHERE c1.code_invoices = t1.id) AS payments_sum,
 
(@invoice_sum_tax-@payments_sum ) AS saldo
 
FROM (t24_invoices AS t1) LEFT JOIN t24_customers AS t2 ON t2.id=t1.code_customers
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i teraz nie działa ani WHERE saldo > 0 ani HAVING > 0

Będę wdzięczny za pomoc w tej kwestii.