Validate if a record exists in a SQL SERVER table

To start you do not have to put quotes in the variables, they already arrive as text, the concatenation is unnecessary.

You can validate if it exists and return a boolean, the cast as bit is to force the bit return and not an int. (causes conflict in some MNOs if not specified)

You could also consider putting this better as a function since it does not perform any modification procedure to the database.

USE [BDCredito]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER PROCEDURE [dbo].[SP_CONS_SOLCUPO_APRO_EXISTE]
(
@Id_Solicitud nvarchar (255),
@CC_Asociado nvarchar (255)
)
AS 
IF EXISTS (SELECT * FROM TBL_DOC_APRO WHERE Id_Solicitud = @Id_Solicitud and CC_Asociado = @CC_Asociado)
    RETURN CAST(1 AS bit);
ELSE 
    RETURN CAST(0 AS bit);

I would only do SELECT COUNT(*) of the table if it returns 0 does not exist if it returns something else means that it exists. If only single records are handled under those conditions, then I can use the response from shop350 if what you want is a bool as a response in your application. Or alternatively you can also do a case if you get many records in the query and convert that answer to a bool . Where @Resultado is an output variable.

First alternative (Returns the number of records that meet the condition):

USE [BDCredito]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER PROCEDURE [dbo].[SP_CONS_SOLCUPO_APRO_EXISTE]
(
@Id_Solicitud nvarchar (255),
@CC_Asociado nvarchar (255),
@Resultado int output
)
AS 
SELECT @Resultado = COUNT(*) FROM TBL_DOC_APRO
WHERE Id_Solicitud = @Id_Solicitud
AND CC_Asociado = @CC_Asociado;

Second alternative (If only one record is obtained as an answer it returns a bool ):

USE [BDCredito]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER PROCEDURE [dbo].[SP_CONS_SOLCUPO_APRO_EXISTE]
(
@Id_Solicitud nvarchar (255),
@CC_Asociado nvarchar (255),
@Resultado bit output
)
AS 
SELECT @Resultado = CAST(COUNT(*) AS BIT) FROM TBL_DOC_APRO
WHERE Id_Solicitud = @Id_Solicitud
AND CC_Asociado = @CC_Asociado;

Third alternative (If several records are obtained a case is made to convert the answer in bool ):

USE [BDCredito]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER PROCEDURE [dbo].[SP_CONS_SOLCUPO_APRO_EXISTE]
(
@Id_Solicitud nvarchar (255),
@CC_Asociado nvarchar (255),
@Resultado bit output
)
AS 
SELECT @Resultado = CAST(CASE WHEN COUNT(*) > 0 THEN 1 ELSE 0 END AS BIT) FROM TBL_DOC_APRO
WHERE Id_Solicitud = @Id_Solicitud
AND CC_Asociado = @CC_Asociado;

Single quotes do not need to be placed.