sql - Why does SELECT DISTINCT return a different number of rows than COUNT(DISTINCT)? -

sql - Why does SELECT DISTINCT return a different number of rows than COUNT(DISTINCT)? -

i noticed unusual on some of informix sql columns (in same table). when query

select distinct colname mytable

i get, example, 40 rows. when do

select count(distinct colname) mytable

i bigger number (like, example, 300). on columns works, (number of rows , count result equal). if this:

select colname mytable

i same number of rows that

select count(*) mytable

gives, yet (much higher) number, nameley exact number of rows in table.

why number returned count (distinct) , row-number returned regular select distinct differ on columns not on others?

tried on myadventureworks. seems working find in sql server 2012.

select count(distinct city) [myadventureworks].[person].[address]

575

select count(city) [myadventureworks].[person].[address]

19614 rows

select distinct city [myadventureworks].[person].[address]

575 rows

sql database count distinct informix