修改表列名：alter table table_name change column old_column_name new_column_name the_type_of_date ;
sql聚集函数：select avg(`column_name`) as avg_column_name from table_name;
avg(distinct ` `)
sum(distinct ` `) as
min(` `) as
max(` `) as
count(` `) as
复合聚集函数 :SELECT MAX(`borrowing balance`) as max_bb,
MIN(`borrowing balance`) as min_bb,
SUM(`borrowing balance`) as sum_bb,
avg(`borrowing balance`) as avg_bb
from loan
where `life of loan`=12