为什么MS SQL Server使用'LIKE'查询返回结果，但使用'='返回空集

Question

I'm using utilizing MS SQL Server 2017. In this example, I have a table [myTable] with one entity full name (varchar(255) .

Inside of this table, I have one record bulk imported with the following parameters

(FIELDTERMINATOR = '~',
 ROWTERMINATOR = '0x0A' )

The following query returns the single record:

SELECT [full name]
FROM myTable
WHERE [full name] LIKE '%LastName%'

However, the following query does not:

SELECT [full name]
FROM myTable
WHERE [full name] = 'Firstname Lastname'

I have verified the record exists, I have copied the value of the entity to make sure there was no funny business going on with the data. It comes up clean, I even tried this:

SELECT [full name]
FROM myTable
WHERE [full name] = 'Firstname Lastname ' 

to make sure there wasn't a random space at the end of the column. What would cause an issue like this?

Answer 1

The = operator works only when it got exact match

but The LIKE operator is used in a to search for a specified pattern in of a data.

so in your case your compare value is not directly matched with the column values that's why it does not returned any rows but in times like it matched pattern so it returned rows

Answer 2

LIKE as the name itself suggests, is mainly used to compare data of some form that form is mostly regular expressions

whereas,

the = finds only the exact match not that it should be of some form rather the data on left of = should be a crystal clear copy of whats on the right of it.

Eg.

As per your above input sample ..where fullname LIKE %FirstName% . This would mean in general, select data Where fullname has firstname in it

If I replace this % ...where fullname LIKE FirstName then it would be same as ...where fullname = FirstName coz here it wants exact match not data of some form

Answer 3

I have a handy script I use to identify invisible characters; it gives you every character in the string but as a separate row per character.

DECLARE @nstring NVARCHAR(1000)

select top 1 @nstring = [full name]
FROM myTable
WHERE [full name] LIKE '%LastName%'

print @nstring

DECLARE @position INT
 SET @position = 1

DECLARE @CharList TABLE (
 Position INT,
 UnicodeChar NVARCHAR(1),
 UnicodeValue INT
 )

WHILE @position <= DATALENGTH(@nstring)
 BEGIN
 INSERT @CharList
 SELECT @position as Position
 ,CONVERT(nchar(1),SUBSTRING(@nstring, @position, 1)) as UnicodeChar
 ,UNICODE(SUBSTRING(@nstring, @position, 1)) as UnicodeValue
 SET @position = @position + 1
 END

SELECT * FROM @CharList

Answer 4

There is a tip given on mssqltips that says you can use SSIS. The article also explains this issue occurring with the ROWTERMINATOR. With that said, I would create an SSIS project that contains a package that does the bulk data insert from the data source. If this is a repeated process it can be scheduled or quickly tested in the SSMS.

If you are new to SSIS or are not familiar with SSIS, refere here: SQL Server Integration Services