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a中没有行，但b中有行（即COUNT(b.id)不是NULL），则此查询返回一行NULL字段的值a以及COUNT(b.id)的{{1}}值tickets_bought。如果表num_rows = 0中没有结果，如何修改此查询以使其不返回行a？

Snap。

query

Answer 1

如果没有GROUP BY子句，MySQL（允许在其他RDBMS中出错的情况）将聚合组应用于b中应该对它们进行分组的所有行。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

以上内容适用于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;

MySQL - 仅当左表中存在行时才加入

2 个答案: