单个查询返回单个数据集。通过查看三个查询，看起来需要三个数据集。您不能通过测量查询来实现相同的功能。所以它看起来应该是好的，除非你改变了你的要求。或者，您可以查看mysql的内部缓存。

//试试这个。。

SELECT * FROM  `meta_holdings` INNER JOIN coins ON coins.cmc_id = meta_holdings.cmc_id WHERE  `userId` = $userId LIMIT 0 , 999 
UNION
SELECT * FROM  `meta_watchlist` INNER JOIN coins ON coins.cmc_id = meta_watchlist.cmc_id WHERE  `userId` = $userId LIMIT 0 , 999
UNION
SELECT * FROM  `coins` ORDER BY  `coins`.`rank` ASC LIMIT 0 , 30

这是一个查询，如果你正在试图找到排名前30位的硬币表数据连同元持有，元观察名单详细信息-

SELECT * FROM  coins 
INNER JOIN meta_holdings ON coins.cmc_id = meta_holdings.cmc_id
INNER JOIN meta_watchlist ON coins.cmc_id = meta_watchlist.cmc_id
WHERE  meta_holdings.userId = $userId and meta_watchlist.userId = $userId
ORDER BY  `coins`.`rank` ASC 
LIMIT 0 , 30