为什么我的查询无法通过Mysql返回准确的结果

Question

I am using following Query to fetch duplicate records from a table.It is working fine for above requirements.

select tc_vessel_name, 
count(tc_vessel_name)as Vessel_Name from t_vessel_m as v
group by tc_vessel_name
having Vessel_Name >1

but the real problem is when i am using below query to find duplicate results having same id entry in other table then it is returning null results. i am not able to understand the real reason behind this.Please help me on this.

SELECT  tc_vessel_id,tc_vessel_name, COUNT(tc_vessel_name) AS Vesse_Name
FROM    t_vessel_m
WHERE   tc_vessel_id IN (SELECT tc_vessel_id FROM t_vessel_x) AND 
t_vessel_m.tc_is_deleted='F'
GROUP BY tc_vessel_name
HAVING ( COUNT(tc_vessel_name) > 1 )

Answer 1

You are not correctly finding duplicates in first query. This:

select tc_vessel_name, 
count(tc_vessel_name)as Vessel_Name from t_vessel_m as v
group by tc_vessel_name
having Vessel_Name > 1

should be this:

select tc_vessel_name, 
count(tc_vessel_name)as Vessel_Name from t_vessel_m as v
group by tc_vessel_name
having count(Vessel_Name) > 1

What you really doing is comparing not count of groups but value of grouping to 1. So first and second statements are different. Look at this fiddle http://sqlfiddle.com/#!9/f5895/1 There are no dups but you get 2 rows as a result. In Sql Server it would give you a cast exception, but it implicitly converts 1 to varchar in MySql .