[SQL] Extending a SQL WHERE clause based on a condition

I want to extend a WHERE condition based on the value of a field. Trying this but t2.field3 is unable to be bound. Is my syntax wrong?

SELECT * FROM table_one t1
JOIN table_two t2 on t1.field = t2.field
WHERE table_one.field2 = "value"
SELECT CASE WHEN t2.field3 IS NOT NULL THEN 'AND t2.field3 = 8'

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