SELECT 123 AS x is the same as SELECT 123 x. Extending to strings, SELECT '123' AS 'x' gives:

MariaDB [test]> SELECT '123' AS 'x';

+-----+

| x   |

+-----+

| 123 |

+-----+

1 row in set (0.000 sec)

Which is what one would expect.
However, SELECT '123' 'x' yields an unexpected result:

MariaDB [test]> SELECT '123' 'x';

+------+

| 123  |

+------+

| 123x |

+------+

1 row in set (0.001 sec)

In the last example, the column name should match the returned value (123x or similar).