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select tp_hoved.id, tp_diffdiag.diffdiag from tp_hoved,tp_diffdiag where tp_hoved.id='$tp_id' AND tp_diffdiag.id='$tp_id'

select tp_hoved.id, tp_diffdiag.diffdiag from tp_hoved LEFT JOIN tp_diffdiag ON (tp_hoved.id=tp_diffdiag.id) WHERE tp_hoved.id='$tp_id'