Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_63231 ( "School" text, "Location" text, "Mascot" text, "County" real, "Enrollment" text, "IHSAA Class" text, "Year Joined" real, "Previous Conference" text )

### Question ###
What is the average year joined of the 277 county?

### Accurate SQL ###

SELECT AVG("Year Joined") FROM table_63231 WHERE "County" = '277'