If there is an index (hence also have statistics on column values) on the Code column, assuming the stats are up to date and accurate, the Optimizer is (rightfully so) more likely to use Seek against that index for the following query because it knows from the statistics that the the index contains much fewer records for Code = 'E'. 

SELECT * FROM Blood_Type WHERE Code = 'E';

If the filter condition instead contain Code = 'F', the Optimizer is more likely to not only ignore the index but instead do a full scan on the table/clustered index!

---

Clean Up

What can you do to fix the skewed CEs?

• Check to make sure there are no missing indexes. You can use one of the famous DMV queries you can search online
• Enable the database settings AUTO_CREATE_STATISTICS and AUTO_UPDATE_STATISTICS 
• Regularly rebuild/refresh your indexes and statistics. Check out the index/statistics maintenance script from Ola Hallengren at https://ola.hallengren.com/
• Keep up with the SQL Server upgrades and patching as they often contain query and optimizer enhancements including how the CE's are calculated and used

/* **** FOR TESTING ONLY IN A NON-PRODUCTION SERVER ****
-- disable the AUTO_CREATE_STATISTICS
USE [master]
GO
ALTER DATABASE [AdminDBA] SET AUTO_CREATE_STATISTICS OFF
GO
*/
set nocount on
if object_id('ce_test_tbl_people') is not null drop table ce_test_tbl_people
go
-- create test table
create table ce_test_tbl_people
(
id int identity primary key,
name varchar(200) not null,
zipcode int not null
);
-- Insert test data
go
insert into ce_test_tbl_people values('ABC', 12345)
GO 100
insert into ce_test_tbl_people values('ABC', 45678)
GO 200
insert into ce_test_tbl_people values('ABC', 93213)
GO 300
insert into ce_test_tbl_people values('ABC', 74616)
GO 400
insert into ce_test_tbl_people values('ABC', 48287)
GO 5000
insert into ce_test_tbl_people values('ABC', 48287)
GO 5000
insert into ce_test_tbl_people values('ABC', 48287)
GO 5000
insert into ce_test_tbl_people values('ABC', 27646)
GO 5
/* 
RUN A QUERY TO TRIGGER INACCURATE CARDINALITY ESTIATES EVENT
MAKE SURE THE XE session for xe_inaccurate_cardinality_estimate is started
*/
SELECT * FROM ce_test_tbl_people WHERE ZIPCODE = 48287;

--  READ THE XE FILEs
;with cte as
(
SELECT top 100 
@@SERVERNAME [SQL Server],
[file_name],
                cast(event_data as xml) event_data
FROM sys.fn_xe_file_target_read_file('xe_inaccurate_cardinality_estimate*.xel', null, null, null)
)
select 
 [SQL Server] 
,dn.v.value('.', 'varchar(100)') [Database]
,ar.v.value('.', 'int') [Actual Rows]
,er.v.value('.', 'int') [Estimated Rows]
,st.v.value('.', 'varchar(8000)') [SQL]
,[file_name] [Event File]
,cte.event_data [Event Data]
,dn.v.value('../@timestamp', 'varchar(100)') [TimeStamp]
,CONCAT('0x', ph.v.value('.', 'varchar(200)')) [Plan Handle]
from cte
CROSS APPLY event_data.nodes('//event/data[@name = "actual_rows"]')     as ar(v)
CROSS APPLY event_data.nodes('//event/data[@name = "estimated_rows"]')  as er(v)
CROSS APPLY event_data.nodes('//event/action[@name = "database_name"]') as dn(v)
CROSS APPLY event_data.nodes('//event/action[@name = "plan_handle"]')   as ph(v)
CROSS APPLY event_data.nodes('//event/action[@name = "sql_text"]')      as st(v)
-- where ar.v.value('.', 'int') > 1000
;
-- select * from sys.dm_exec_cached_plans where plan_handle = 0x303630303031303032393830376432613630353362656434386430313030

-- DROP THE XE xe_inaccurate_cardinality_estimate
IF exists (SELECT * FROM sys.server_event_sessions where name = 'xe_inaccurate_cardinality_estimate')
BEGIN
IF exists(select * from sys.dm_xe_sessions where name = 'xe_inaccurate_cardinality_estimate')
ALTER EVENT SESSION [xe_inaccurate_cardinality_estimate] ON SERVER STATE=STOP;
DROP EVENT SESSION [xe_inaccurate_cardinality_estimate] ON SERVER 
END

$sql_instance_name = "TestSQLServerTestInstance"
$sql_query = "SELECT distinct @@SERVERNAME [SQL_Instance],
                SERVERPROPERTY('MachineName') [Computer_Name],
                [file_name]
FROM sys.fn_xe_file_target_read_file('xe_inaccurate_cardinality_estimate*.xel', null, null, null);
"
#If you are using a SQL login with password, at the end of the Invoke-Sqlcmd add: -Username '<SQL Login>' -Password '<password>'
$sql_xe_files      = Invoke-Sqlcmd -ServerInstance $sql_instance_name -Query $sql_query
$sql_xe_file_names = $sql_xe_files.file_name
$sql_host_name     = $sql_xe_files.Computer_Name[1]
$sql_host_fqdn     = [System.Net.Dns]::GetHostEntry($sql_host_name).HostName
$file_object = Invoke-Command -ArgumentList $sql_xe_file_name -computername $sql_host_fqdn -scriptBlock {get-item $Using:sql_xe_file_names}
#Uncomment the next line to actually remove the event files on the server
#$file_object = Invoke-Command -ArgumentList $sql_xe_file_name -computername $sql_host_fqdn -scriptBlock {remove-item $Using:sql_xe_file_names}