SHOW is not a substitute for SELECT or vice versa.
SELECT column_name FROM table_name will select all records from column_name column so you basically get values from that column and from all records.
SHOW column_name FROM table_name won't work because it doesn't work for such purpose as SELECT.
You would get a syntax error if you try to execute it. SHOW only provides useful information about columns, tables or databases or about server. For example: SHOW columns FROM table_name; will fetch you all the information about all columns of table_name, it will show you all the properties that each column has.
You can find out more about SHOW in MYSQL official documentation website.