参考地址：http://knexjs.org/
原始mysql语句：
SELECT
a.*, (
SELECT
b. NAME
FROM
kind b
WHERE
b.id = a.kind
) AS kind_name,
(
SELECT
c.url
FROM
music c
WHERE
c.id = a.music
) AS music_url
FROM
card a
WHERE
a. SHOW = 1转化为knex语句：
module.exports = async ctx => {
ctx.body = await knex('card')
.join('kind', 'card.kind', '=', 'kind.id')
.join('music', 'card.music', '=', 'music.id')
.select('card.*', 'kind.name as kind_name', 'music.url as music_url')
.where('show', 1)
}编译后的sql语句：
SELECT
`card`.*,
`kind`.`name` AS `kind_name`,
`music`.`url` AS `music_url`
FROM
`card`
INNER JOIN `kind` ON `card`.`kind` = `kind`.`id`
INNER JOIN `music` ON `card`.`music` = `music`.`id`
WHERE
`show` = 1
