A função PARSENAME() é o oposto de QUOTENAME()

Baseado no comentário de ypercubeᵀᴹ que diz:

...você pode encontrar uma string xque parsename(quotename(x),1)seja diferente de x? Se não, isso se enquadra na definição de "função inversa" ;) ...

... Criei uma instrução simples para dar uma olhada nos diferentes valores retornados por essas funções. O código é o seguinte:

Código

DECLARE 
-- Text that I will be converting using QUOTENAME()
@Text2Quote_1 AS NVARCHAR(20),
@Text2Quote_2 AS NVARCHAR(20),
@Text2Quote_3 AS NVARCHAR(20),
@Text2Quote_4 AS NVARCHAR(20),
-- The characters used for "quotation"
@QuoteChar_1 AS NCHAR(1),
@QuoteChar_2 AS NCHAR(1),
@QuoteChar_3 AS NCHAR(1),
-- The Parsing option as defined in the original MS documnenation for PARSSENAME()
@ParseParam AS INT

SET @Text2Quote_1 = N'Test'
SET @Text2Quote_2 = N'[Test]'
SET @Text2Quote_3 = N'Test.dbo.test'
SET @Text2Quote_4 = N'[Test].[dbo].[test]'

SET @QuoteChar_1 = ''''
SET @QuoteChar_2 = '[' 
SET @QuoteChar_3 = '"' 

SET @ParseParam = 1 -- Parsing level : Object name
-- checking the results for ' single quotes
SELECT  @Text2Quote_1 AS OriginalText_1,
        @Text2Quote_2 AS OriginalText_2,
        @Text2Quote_3 AS OriginalText_3,
        @Text2Quote_4 AS OriginalText_4
SELECT  QUOTENAME(@Text2Quote_1,@QuoteChar_1) AS QuotedText_1, 
        QUOTENAME(@Text2Quote_2,@QuoteChar_1) AS QuotedText_2, 
        QUOTENAME(@Text2Quote_3,@QuoteChar_1) AS QuotedText_3, 
        QUOTENAME(@Text2Quote_4,@QuoteChar_1) AS QuotedText_4
select  PARSENAME(QUOTENAME(@Text2Quote_1,@QuoteChar_1),@ParseParam) AS Parsed_QuotedText_1, 
        PARSENAME(QUOTENAME(@Text2Quote_2,@QuoteChar_1),@ParseParam) AS Parsed_QuotedText_2,
        PARSENAME(QUOTENAME(@Text2Quote_3,@QuoteChar_1),@ParseParam) AS Parsed_QuotedText_3,
        PARSENAME(QUOTENAME(@Text2Quote_4,@QuoteChar_1),@ParseParam) AS Parsed_QuotedText_4

Basicamente, defino valores de string, produzo esse valor QUOTENAME()e produzo o valor, depois PARSENAME()o QUOTENAME()valor ed e o produzo.

Resultados

Os resultados foram bastante interessantes:

OriginalText_1           OriginalText_2           OriginalText_3           OriginalText_4          
------------------------ ------------------------ ------------------------ ------------------------
Test                     [Test]                   Test.dbo.test            [Test].[dbo].[test]     

(1 row(s) affected)                                                                                

QuotedText_1             QuotedText_2             QuotedText_3             QuotedText_4            
------------------------ ------------------------ ------------------------ ------------------------
'Test'                   '[Test]'                 'Test.dbo.test'          '[Test].[dbo].[test]'   

(1 row(s) affected)                                                                                

Parsed_QuotedText_1      Parsed_QuotedText_2      Parsed_QuotedText_3      Parsed_QuotedText_4     
------------------------ ------------------------ ------------------------ ------------------------
'Test'                   NULL                     test'                    NULL                    

(1 row(s) affected)                                                                                

Quando o texto base Test.dbo.test( OriginalText_3 ) é passado para a QUOTENAME()função, ele é convertido em: 'Test.dbo.test'( QuotedText_3 )

Quando a string QuotedText_3'Test.dbo.test' é passada para a PARSENAME()função, ela é convertida em: test'( Parsed_QuotedText_3 )

Conclusão

Visto que provei a tese de ypercubeᵀᴹ errada, acho que é seguro afirmar que a função PARSENAME()não é a inversa de QUOTENAME().

Com base no feedback de Aaron Bertrand aqui o código para colchetes (adicionar ao script original)[ ]

Código adicional para colchetes[ ]

-- checking the results for [ brackets                                                                           
SELECT  @Text2Quote_1 AS OriginalText_1,                                                                         
        @Text2Quote_2 AS OriginalText_2,                                                                         
        @Text2Quote_3 AS OriginalText_3,                                                                         
        @Text2Quote_4 AS OriginalText_4                                                                          
SELECT  QUOTENAME(@Text2Quote_1,@QuoteChar_2) AS QuotedText_1,                             
        QUOTENAME(@Text2Quote_2,@QuoteChar_2) AS QuotedText_2,                             
        QUOTENAME(@Text2Quote_3,@QuoteChar_2) AS QuotedText_3,                             
        QUOTENAME(@Text2Quote_4,@QuoteChar_2) AS QuotedText_4                              
select  PARSENAME(QUOTENAME(@Text2Quote_1,@QuoteChar_2),@ParseParam) AS Parsed_QuotedText_1,
        PARSENAME(QUOTENAME(@Text2Quote_2,@QuoteChar_2),@ParseParam) AS Parsed_QuotedText_2,
        PARSENAME(QUOTENAME(@Text2Quote_3,@QuoteChar_2),@ParseParam) AS Parsed_QuotedText_3,
        PARSENAME(QUOTENAME(@Text2Quote_4,@QuoteChar_2),@ParseParam) AS Parsed_QuotedText_4

Saída para colchetes[ ]

OriginalText_1           OriginalText_2           OriginalText_3           OriginalText_4
------------------------ ------------------------ ------------------------ ------------------------
Test                     [Test]                   Test.dbo.test            [Test].[dbo].[test]

(1 row(s) affected)

QuotedText_1             QuotedText_2             QuotedText_3             QuotedText_4
------------------------ ------------------------ ------------------------ ------------------------
[Test]                   [[Test]]]                [Test.dbo.test]          [[Test]].[dbo]].[test]]]

(1 row(s) affected)

Parsed_QuotedText_1      Parsed_QuotedText_2      Parsed_QuotedText_3      Parsed_QuotedText_4
------------------------ ------------------------ ------------------------ ------------------------
Test                     [Test]                   Test.dbo.test            [Test].[dbo].[test]

(1 row(s) affected)

e para completar as coisas o código para as aspas duplas

Código adicional para aspas duplas"

-- checking the results for [ brackets                                                                           
SELECT  @Text2Quote_1 AS OriginalText_1,                                                                         
        @Text2Quote_2 AS OriginalText_2,                                                                         
        @Text2Quote_3 AS OriginalText_3,                                                                         
        @Text2Quote_4 AS OriginalText_4                                                                          
SELECT  QUOTENAME(@Text2Quote_1,@QuoteChar_3) AS QuotedText_1,                             
        QUOTENAME(@Text2Quote_2,@QuoteChar_3) AS QuotedText_2,                             
        QUOTENAME(@Text2Quote_3,@QuoteChar_3) AS QuotedText_3,                             
        QUOTENAME(@Text2Quote_4,@QuoteChar_3) AS QuotedText_4                              
select  PARSENAME(QUOTENAME(@Text2Quote_1,@QuoteChar_3),@ParseParam) AS Parsed_QuotedText_1,
        PARSENAME(QUOTENAME(@Text2Quote_2,@QuoteChar_3),@ParseParam) AS Parsed_QuotedText_2,
        PARSENAME(QUOTENAME(@Text2Quote_3,@QuoteChar_3),@ParseParam) AS Parsed_QuotedText_3,
        PARSENAME(QUOTENAME(@Text2Quote_4,@QuoteChar_3),@ParseParam) AS Parsed_QuotedText_4

Saída para aspas duplas"

OriginalText_1           OriginalText_2           OriginalText_3           OriginalText_4
------------------------ ------------------------ ------------------------ ------------------------
Test                     [Test]                   Test.dbo.test            [Test].[dbo].[test]

(1 row(s) affected)

QuotedText_1             QuotedText_2             QuotedText_3             QuotedText_4
------------------------ ------------------------ ------------------------ ------------------------
"Test"                   "[Test]"                 "Test.dbo.test"          "[Test].[dbo].[test]"

(1 row(s) affected)

Parsed_QuotedText_1      Parsed_QuotedText_2      Parsed_QuotedText_3      Parsed_QuotedText_4
------------------------ ------------------------ ------------------------ ------------------------
Test                     [Test]                   Test.dbo.test            [Test].[dbo].[test]

(1 row(s) affected)

Com base no feedback do ypercube™ e no novo feedback de Aaron Bertrand , chegamos a uma conclusão mútua

PARSENAME()é o inverso de QUOTENAME()se as seguintes condições forem atendidas:

PARSENAME(x,1)-1(QUOTENAME(x,q))onde q={1,2}ex='{'any_string_value'}

Referências:

• PARSENAME (Transact SQL) (Microsoft Docs)
• QUOTENAME (Transact SQL) (Microsoft Docs)