res_resource_catalog表对于res_info_item表是一对多, 查询res_resource_catalog信息的同时,统计res_info_item中该条res_resource_catalog数据对应的数量

GROUP BY rrc.id是必须有的,如果没有的话结果中将永远只有一条数据

COUNT(DISTINCT rii.id) AS itemCount中如果不加 DISTINCT 结果中 itemCount 和 applyCount 结果将相同,这是错误的, 但是如果只有一个count可以不加

SELECT
    rrc.id,
    rrc.resource_name,
    COUNT(DISTINCT rii.id) AS itemCount,
    COUNT(DISTINCT rra.id) AS applyCount
FROM
    res_resource_catalog AS rrc
LEFT JOIN res_info_item rii ON rrc.id = rii.resource_id
LEFT JOIN res_resource_apply rra ON rrc.id = rra.resource_id
GROUP BY rrc.id