MySQL, I need a query to show all the Amino_acid_id symbols (a5,a8) that contain only one type of letter, so for example the first one below a5 contains the Codon_sequence TTT.

Position1 Codon_id Codon_sequence TTT Position2 T Position3 Amino_acid_id codon_fk T a5 1 T 10 ATC A T a8 11 T A a8 c12 ATG A T a11 c13 GTT G T T a18 14 GTC G T a18 15 GTA G T A a18 16 GTG G T G a18 17 T T a16 c18 cc T C . a16 19 TCA T A a16 2 T T a5 C20 TCG T G a16 c21 T a13 c22 cc a13 c23 A a13 24 CCG G a13 25 T a17 26 a17 27 A A a17