Sql Case语句何时不为null

SELECT 
    CASE 
        WHEN condition1 THEN result1
        WHEN condition2 THEN result2
        ELSE default_result
    END
FROM table_name;

SELECT column1, column2,
    CASE
        WHEN column1 > column2 THEN 'Column1 is greater'
        ELSE 'Column2 is greater'
    END AS comparison_result
FROM table_name;

SELECT column1, column2,
    (CASE
        WHEN column1 > column2 THEN 'Column1 is greater'
        ELSE 'Column2 is greater'
    END) AS comparison_result
FROM table_name;