SQL Server 2019 (Linux)

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CREATE TABLE USERS(
 FIRSTNAME VARCHAR(50), 
 LASTNAME VARCHAR(50)
);



INSERT INTO USERS VALUES ('JOHN','SMITH');
INSERT INTO USERS VALUES ('DAVID','BECKHAM');
INSERT INTO USERS VALUES ('ELON','MUSK');
3 rows affected

;WITH CTE AS (
  SELECT *
  FROM USERS
  WHERE FIRSTNAME = 'JANE'
)
SELECT FIRSTNAME,LASTNAME
FROM CTE 
UNION ALL
SELECT FIRSTNAME,LASTNAME
FROM USERS 
WHERE NOT EXISTS (
   SELECT 1
   FROM CTE
)


FIRSTNAME
LASTNAME


JOHN
SMITH

DAVID
BECKHAM

ELON
MUSK


;WITH CTE AS (
  SELECT *
  FROM USERS
  WHERE FIRSTNAME = 'JOHN'
)
SELECT FIRSTNAME,LASTNAME
FROM CTE 
UNION ALL
SELECT FIRSTNAME,LASTNAME
FROM USERS 
WHERE NOT EXISTS (
   SELECT 1
   FROM CTE
)


FIRSTNAME
LASTNAME


JOHN
SMITH