-- SQL Server will perform the filter and the conversion in either order

-- Note that Compute Scalar is plonked anywhere convenient in the execution plan

-- and may not relate to where it actually occurs, as here.

WITH SampleData AS (SELECT * FROM (VALUES ('1.2.1'), ('1'), ('1.1'), ('1.1.1')) V (S))

SELECT

s1.s,

NewVal = CASE WHEN s1.s NOT LIKE '%.%.%' THEN CONVERT(decimal(2,1),s1.s) ELSE NULL END

FROM SampleData s1, SampleData s2, SampleData s3

WHERE s1.s NOT LIKE '%.%.%' OR s2.s NOT LIKE '%.%.%';

WITH SampleData AS (SELECT * FROM (VALUES ('1.2.1'), ('1'), ('1.1'), ('1.1.1')) V (S))

SELECT

s1.s,

CONVERT(decimal(2,1),s1.s)

FROM SampleData s1, SampleData s2, SampleData s3

WHERE s1.s NOT LIKE '%.%.%' OR s2.s NOT LIKE '%.%.%';