user: Question: What is the notion when the crystal structure is Tetragonal and the formula is Bi 2 Sr 2 CaCu 2 O 8
Columns: ['Formula', 'Notation', 'T c (K)', 'No. of Cu-O planes in unit cell', 'Crystal structure']
Types: ['text', 'text', 'real', 'real', 'text']
Sample row: ['YBa 2 Cu 3 O 7', '123', 92.0, 2.0, 'Orthorhombic']
SQL:

assistant: SELECT "Notation" FROM "Table" WHERE "Crystal structure" = 'Tetragonal' AND "Formula" = 'Bi 2 Sr 2 CaCu 2 O 8';