mysql - MySQL - 仅当左表中存在一行时才加入

Question

这是MySQL：

SELECT  a.id,
        a.name,
        a.n,
        a.r,
        a.pot,
        a.ticket_price,
        a.starting_tickets,
        a.started,
        a.end,
        COUNT(b.id) tickets_bought 
FROM current_lotteries a
   JOIN lottery_tickets b ON b.lid=a.id
WHERE a.cid=1 
ORDER BY started DESC LIMIT 1    

在搜索中，如果没有行 froma但有行 in b（即COUNT(b.id)is not NULL），则此查询返回一行，其中NULL包含a字段值以及COUNT(b.id)as的值tickets_bought。num_rows = 0如果表中没有结果，如何修改此查询使其不返回行 ( ) a？

一个快照。

Answer 1

如果没有子句，MySQL（它允许在其他 RDBMS 中出现错误的情况下这样做）在应该对它们进行分组时GROUP BY将聚合组应用于所有行。b添加GROUP BY a.id

SELECT  a.id,
        a.name,
        a.n,
        a.r,
        a.pot,
        a.ticket_price,
        a.starting_tickets,
        a.started,
        a.end,
        COUNT(b.id) tickets_bought 
FROM current_lotteries a
   JOIN lottery_tickets b ON b.lid=a.id
WHERE a.cid=1 
GROUP BY a.id
ORDER BY started DESC LIMIT 1    

以上将适用于 MySQL，但不适用于其他地方。更便携的版本使用相关子查询：

SELECT  a.id,
        a.name,
        a.n,
        a.r,
        a.pot,
        a.ticket_price,
        a.starting_tickets,
        a.started,
        a.end,
        b.tickets_bought
FROM current_lotteries a
        /* More portable to join against a subquery which returns the count per group */
        JOIN (
            SELECT b.lid, COUNT(*) AS tickets_bought 
            FROM lottery_tickets 
            GROUP BY lid
        ) b ON a.id = b.lid
WHERE a.cid = 1
ORDER BY started DESC LIMIT 1

Answer 2

试试这个：

SELECT a.id, a.name, a.n, a.r, a.pot, a.ticket_price, 
       a.starting_tickets, a.started, a.end, b.tickets_bought 
FROM current_lotteries a 
RIGHT JOIN (SELECT b.lid, COUNT(*) AS tickets_bought 
            FROM lottery_tickets GROUP BY lid ) b ON a.id = b.lid 
WHERE a.cid = 1 
ORDER BY started DESC 
LIMIT 1;