sql
SELECT user_id, MAX(time_stamp) AS last_stamp
FROM Logins
WHERE YEAR(time_stamp) = 2020
GROUP BY user_id;

SELECT user_id, MAX(time_stamp) as last_stamp
FROM Logins
WHERE TIMEDIFF(time_stamp,'2020-01-01 00:00:00')>0 
    and TIMEDIFF(time_stamp,'2020-12-31 23:59:59') <0
GROUP BY user_id


SELECT user_id, MAX(time_stamp) as last_stamp
FROM Logins
WHERE DATEDIFF(time_stamp,'2020-01-01 00:00:00')>=0 
    and DATEDIFF(time_stamp,'2020-12-31 23:59:59') <=0
GROUP BY user_id

二

sql
SELECT 'Low Salary' AS category, COUNT(*) AS accounts_count
FROM Accounts
WHERE income < 20000
UNION
SELECT 'Average Salary' AS category, COUNT(*) AS accounts_count
FROM Accounts
WHERE income >= 20000 AND income <= 50000
UNION
SELECT 'High Salary' AS category, COUNT(*) AS accounts_count
FROM Accounts
WHERE income > 50000;




SELECT 
    'Low Salary' AS category,
    SUM(CASE WHEN income < 20000 THEN 1 ELSE 0 END) AS accounts_count
FROM 
    Accounts
    
UNION
SELECT  
    'Average Salary' category,
    SUM(CASE WHEN income >= 20000 AND income <= 50000 THEN 1 ELSE 0 END) 
    AS accounts_count
FROM 
    Accounts

UNION
SELECT 
    'High Salary' category,
    SUM(CASE WHEN income > 50000 THEN 1 ELSE 0 END) AS accounts_count
FROM 
    Accounts