您没有按正确的列排序。如果需要最新日期的值，则需要按日期和时间列降序排列：

SELECT m.merchantname as MerchantName,
         COALESCE(COUNT(t.transactionid), 0) as NoofTransaction, 
       COALESCE(SUM(t.transactionamount), 0) as TotalAmount,
       (SELECT s.statementbalance
        FROM statement s
        WHERE m.merchantid = s.merchantid
        AND s.statementdate <= '2018-01-16'
        ORDER BY s.statementdate DESC, s.statementtime DESC
        LIMIT 1 
       ) as BalanceRemaining
FROM merchant m 
LEFT JOIN transaction t ON m.merchantid = t.merchantid
                       AND t.transactiondate = '2018-01-16'
GROUP BY m.merchantid
ORDER BY m.merchantid ASC;

要启用此功能，还需要更改数据的关联方式，而不是按transactionid，而是按merchantid。

| MerchantName | NoofTransaction | TotalAmount | BalanceRemaining |
|--------------|-----------------|-------------|------------------|
|         Dog1 |               0 |           0 |           (null) |
|         Cat2 |              13 |         115 |            24.16 |
|      Parrot3 |               1 |          20 |           299.25 |
|      Beaver4 |               0 |           0 |           (null) |

请参见：http://sqlfiddle.com/#!9/cc2440/3号

如果你想要“7”，也许你想要一个 ASC 排序而不是 DESC 排序。
我还建议使用表别名：

SELECT m.merchantname as MerchantName,
       COUNT(t.transactionid), 0) as NoofTransaction, 
       COALESCE(SUM(t.transactionamount), 0) as TotalAmount,
       (SELECT s.statementbalance
        FROM statement s
        WHERE t.transactionid = s.transactionid
        ORDER BY statementbalance ASC
        LIMIT 1 
       ) as BalanceRemaining
FROM merchant m LEFT JOIN
     transaction t
     ON m.merchantid = t.merchantid AND t.transactiondate = '2018-01-16'
GROUP BY m.merchantid
ORDER BY m.merchantid ASC;

笔记：
表别名使查询更易于编写和读取。
仅对字符串和日期常量使用单引号。不要将它们用作列别名。
我推荐 as 列别名。 COUNT() 永不回头 NULL ，所以 COALESCE() 是不必要的。