A, count (*), count ( 1), count ( field names) difference
select count (*) from t_md_inst --153797 - contains fields for the null record
select count (1) from t_md_inst --- 153797 - comprising a recording field is null
select count (class_id) from t_md_inst --19883 - does not contain a null record class_id
select count (distinct class_id) from t_md_inst --29 --- class_id duplicate records are not displayed
二、group by + having + order by
select quote_class, quoteed_type
from t_meta_quote
group by quote_class, quoteed_type
having quote_class != 'DDIndex'
order by quoteed_type desc, quote_class asc;
Three, || connector: connecting string field and
Fourth, to two distinct fields of heavy
Fifth, the pseudo-column, alias, is null, like
