The Mexico-US border is in fact 1,954 miles (3,145km) long.
Since the start of 2018, more than 38,000 migrants have arrived in Spain, mostly by sea.
Spain has no sovereignty over the Sahara, but it does possess two small enclaves on the north African coast, Ceuta and Melilla, separated from Morocco by controversial wire fences.
However, the former president of the European Parliament said that European countries do need to urgently work towards a solution because Africa's population is set to double in the next 20 years (the World Economic Forum says the doubling will actually take around 32 years).
Trump's call for a wall on the southern border - which he repeatedly insisted that Mexico would pay for - was a hallmark of his presidential campaign, with "Build the Wall!" chants still breaking out at his frequent "MAGA" rallies.
Earlier this month, Trump said he was considering taking drastic military measures to build the wall instead of relying on Congress.