UPDATE plt_stock PS
    JOIN (
        SELECT c_product_sku FROM plt_stock WHERE ifnull(c_synced,'') != 'T' GROUP BY c_product_sku
        ) PSF ON PS.c_product_sku = PSF.c_product_sku
    LEFT JOIN plv_item_simple PIS ON PS.c_product_sku = PIS.c_product_sku AND ifnull(PIS.c_product_sku,'') != ''
SET c_synced = 'T', c_error_msg = 'Product SKU does not exists'
  WHERE PIS.c_product_sku IS NULL

UPDATE plt_stock PS
LEFT JOIN plv_item_simple PIS ON PS.c_product_sku = PIS.c_product_sku AND ifnull(PIS.c_product_sku,'') != ''
    SET c_synced = 'T', c_error_msg = 'Product SKU does not exists'
        WHERE PIS.c_product_sku IS NULL