选择LIKE或CHARINDEX（）函数以使用SQL查找一些特殊的Unicode字符

Question

What wrong with my second and third query that did not output any record, while the first one outputs one record.

Query 1:

DECLARE @FullName nvarchar(50)
DECLARE @FULLNamelike nvarchar(50)
SET @FullName = 'NGUYỄN THỊ DIỆU'
SET @FULLNamelike = N'%' + @FullName + '%'

SELECT HR_CInformation.[CandidateId]
    ,HR_CInformation.[FullName]
    ,HR_CInformation.[Email]
FROM [dbo].[IT_HR_Candidate_Information] HR_CInformation
WHERE (HR_CInformation.[FullName] like N'%NGUYỄN THỊ DIỆU%' OR @FULLNamelike = '')

• Result: 21 NGUYỄN THỊ DIỆU addieunt@gmail.com

Query 2:

DECLARE @FullName nvarchar(50)
DECLARE @FULLNamelike nvarchar(50)
SET @FullName = 'NGUYỄN THỊ DIỆU'
SET @FULLNamelike = N'%' + @FullName + '%'

SELECT HR_CInformation.[CandidateId]
    ,HR_CInformation.[FullName]
    ,HR_CInformation.[Email]
FROM [dbo].[IT_HR_Candidate_Information] HR_CInformation
WHERE (HR_CInformation.[FullName] like @FULLNamelike OR @FULLNamelike = '')

Query 3:

DECLARE @FullName nvarchar(50)
DECLARE @FULLNamelike nvarchar(50)
SET @FullName = 'NGUYỄN THỊ DIỆU'
SET @FULLNamelike = N'%' + @FullName + '%'

SELECT HR_CInformation.[CandidateId]
    ,HR_CInformation.[FullName]
    ,HR_CInformation.[Email]
FROM [dbo].[IT_HR_Candidate_Information] HR_CInformation
WHERE (CHARINDEX(@FULLNamelike, HR_CInformation.[FullName]) > 0 OR @FULLNamelike = '')

Answer 1

It's because your unicode characters are converted into question marks.

Thus, indeed you need to use N when you declare and assign a unicode value to your variable FullName

Try this query to clarify:

DECLARE @FullName nvarchar(50)
DECLARE @FullNameO nvarchar(50)
DECLARE @FullNamelike nvarchar(50)
DECLARE @FullNameOkay nvarchar(50)
SET @FullName = 'NGUYỄN THỊ DIỆU'
SET @FullNameO = N'NGUYỄN THỊ DIỆU'
SET @FullNamelike = N'%' + @FullName +'%'
SET @FullNameOkay = N'%' + @FullNameO +'%'
SELECT @FullNamelike, N'%NGUYỄN THỊ DIỆU%', @FullNameOkay

SQLFiddle: http://sqlfiddle.com/#!3/d41d8/40582