如何用sql分组

我在一个结果集：cid cname aid
29e629e6-126971c698f-3f7993f935df85b970fa5d1b3ea42690 资料下载 259c259c-126b0d9e7bc-5068e670f0a6e5973d021535d0a1bc2e
29e629e6-126971c698f-3f7993f935df85b970fa5d1b3ea42690 资料下载 259c259c-126b16264d6-5068e670f0a6e5973d021535d0a1bc2e
29e629e6-126971bf143-3f7993f935df85b970fa5d1b3ea42690 政策法规 259c259c-126b1645a3e-5068e670f0a6e5973d021535d0a1bc2e
29e629e6-126971c45cb-3f7993f935df85b970fa5d1b3ea42690 问题解答 259c259c-126b185b6f3-5068e670f0a6e5973d021535d0a1bc2e
29e629e6-126971b9613-3f7993f935df85b970fa5d1b3ea42690 最新动态 15c815c8-12756b6ec50-5068e670f0a6e5973d021535d0a1bc2e
29e629e6-126971c698f-3f7993f935df85b970fa5d1b3ea42690 资料下载 600a600a-127a4aad930-5068e670f0a6e5973d021535d0a1bc2e
29e629e6-126971b9613-3f7993f935df85b970fa5d1b3ea42690 最新动态 600a600a-127ae00ecfc-5068e670f0a6e5973d021535d0a1bc2e
29e629e6-126971b9613-3f7993f935df85b970fa5d1b3ea42690 最新动态 600a600a-12806aa2c4a-5068e670f0a6e5973d021535d0a1bc2e
57585758-12ad080b82a-b136e347d4df51a64c12dce88c0bfa2c 资料下载 null
39203920-12a698b9902-b136e347d4df51a64c12dce88c0bfa2c 资料下载 null
7960796-12740e6757c-b136e347d4df51a64c12dce88c0bfa2c 工作动态 null
7960796-12740e6fc8e-b136e347d4df51a64c12dce88c0bfa2c 经验交流 null
7960796-12740e7f507-b136e347d4df51a64c12dce88c0bfa2c 问题解答 null
57585758-12ad0804a1e-b136e347d4df51a64c12dce88c0bfa2c 审计动态 null
300a300a-123a68ca826-5068e670f0a6e5973d021535d0a1bc2e 典型SQL语句 null
7960796-12740e6ca14-b136e347d4df51a64c12dce88c0bfa2c 财经法规 null
53ba53ba-1227878fb84-b136e347d4df51a64c12dce88c0bfa2c 经验交流 null
39203920-12a698b29fc-b136e347d4df51a64c12dce88c0bfa2c 政策法规 null
57585758-12aada2782b-b136e347d4df51a64c12dce88c0bfa2c 审计动态 null
39203920-12a698b6ef4-b136e347d4df51a64c12dce88c0bfa2c 审计动态 null
4a3c4a3c-12718b657fa-3f7993f935df85b970fa5d1b3ea42690 资料下载 null
4a3c4a3c-12718b5dcee-3f7993f935df85b970fa5d1b3ea42690 问题答疑 null
53ba53ba-122787879d1-b136e347d4df51a64c12dce88c0bfa2c 政策法规 null
13b613b6-12ab0e4ca5a-3f7993f935df85b970fa5d1b3ea42690 问题答疑 null
57585758-12ad0812972-b136e347d4df51a64c12dce88c0bfa2c 问题答疑 null
39203920-12a698bbeda-b136e347d4df51a64c12dce88c0bfa2c 经验交流 null
53ba53ba-1227878c438-b136e347d4df51a64c12dce88c0bfa2c 资料下载 null
300a300a-123a68d6b66-5068e670f0a6e5973d021535d0a1bc2e 专家经验 null
57585758-12ad080f6e9-b136e347d4df51a64c12dce88c0bfa2c 经验交流 null
4a3c4a3c-12718b613cd-3f7993f935df85b970fa5d1b3ea42690 政策法规 null
57585758-12ad080015d-b136e347d4df51a64c12dce88c0bfa2c 政策法规 null
57585758-12aada2a47b-b136e347d4df51a64c12dce88c0bfa2c 资料下载 null
54345434-1237dea04ac-5068e670f0a6e5973d021535d0a1bc2e 资料下载 （自查） null
4a3c4a3c-12718b55222-3f7993f935df85b970fa5d1b3ea42690 审计动态 null
300a300a-123a68bafbc-5068e670f0a6e5973d021535d0a1bc2e 最新动态 null
53ba53ba-122eeaa5cbd-b136e347d4df51a64c12dce88c0bfa2c 问题答复 null
44644464-124990ba71c-3f7993f935df85b970fa5d1b3ea42690 公示公告 null
7960796-12740e69b63-b136e347d4df51a64c12dce88c0bfa2c 案件披露 null
54345434-1237ddc95d7-5068e670f0a6e5973d021535d0a1bc2e 政策法规（自查）  null
300a300a-123a68cec43-5068e670f0a6e5973d021535d0a1bc2e 审计案例 null
29e629e6-126971c9d71-3f7993f935df85b970fa5d1b3ea42690 经验交流 null
57585758-12aada2dfaf-b136e347d4df51a64c12dce88c0bfa2c 经验交流 null
4a3c4a3c-12718b59527-3f7993f935df85b970fa5d1b3ea42690 经验交流 null
7960796-12740e6374a-b136e347d4df51a64c12dce88c0bfa2c 项目公告 null
57585758-12aada23eeb-b136e347d4df51a64c12dce88c0bfa2c 政策法规 null
300a300a-123a68c465e-5068e670f0a6e5973d021535d0a1bc2e 答疑解惑 null
[b]中进行分组，如果group by cid 那么会得到：
cid num
29e629e6-126971c698f-3f7993f935df85b970fa5d1b3ea42690 3
29e629e6-126971bf143-3f7993f935df85b970fa5d1b3ea42690 1
29e629e6-126971c45cb-3f7993f935df85b970fa5d1b3ea42690 1
29e629e6-126971b9613-3f7993f935df85b970fa5d1b3ea42690 3
57585758-12ad080b82a-b136e347d4df51a64c12dce88c0bfa2c 1
39203920-12a698b9902-b136e347d4df51a64c12dce88c0bfa2c 1
7960796-12740e6757c-b136e347d4df51a64c12dce88c0bfa2c 1
7960796-12740e6fc8e-b136e347d4df51a64c12dce88c0bfa2c 1
7960796-12740e7f507-b136e347d4df51a64c12dce88c0bfa2c 1
57585758-12ad0804a1e-b136e347d4df51a64c12dce88c0bfa2c 1
300a300a-123a68ca826-5068e670f0a6e5973d021535d0a1bc2e 1
7960796-12740e6ca14-b136e347d4df51a64c12dce88c0bfa2c 1
53ba53ba-1227878fb84-b136e347d4df51a64c12dce88c0bfa2c 1
39203920-12a698b29fc-b136e347d4df51a64c12dce88c0bfa2c 1
57585758-12aada2782b-b136e347d4df51a64c12dce88c0bfa2c 1
39203920-12a698b6ef4-b136e347d4df51a64c12dce88c0bfa2c 1
4a3c4a3c-12718b657fa-3f7993f935df85b970fa5d1b3ea42690 1
4a3c4a3c-12718b5dcee-3f7993f935df85b970fa5d1b3ea42690 1
53ba53ba-122787879d1-b136e347d4df51a64c12dce88c0bfa2c 1
13b613b6-12ab0e4ca5a-3f7993f935df85b970fa5d1b3ea42690 1
57585758-12ad0812972-b136e347d4df51a64c12dce88c0bfa2c 1
39203920-12a698bbeda-b136e347d4df51a64c12dce88c0bfa2c 1
53ba53ba-1227878c438-b136e347d4df51a64c12dce88c0bfa2c 1
300a300a-123a68d6b66-5068e670f0a6e5973d021535d0a1bc2e 1
57585758-12ad080f6e9-b136e347d4df51a64c12dce88c0bfa2c 1
4a3c4a3c-12718b613cd-3f7993f935df85b970fa5d1b3ea42690 1
57585758-12ad080015d-b136e347d4df51a64c12dce88c0bfa2c 1
57585758-12aada2a47b-b136e347d4df51a64c12dce88c0bfa2c 1
54345434-1237dea04ac-5068e670f0a6e5973d021535d0a1bc2e 1
4a3c4a3c-12718b55222-3f7993f935df85b970fa5d1b3ea42690 1
300a300a-123a68bafbc-5068e670f0a6e5973d021535d0a1bc2e 1
53ba53ba-122eeaa5cbd-b136e347d4df51a64c12dce88c0bfa2c 1
44644464-124990ba71c-3f7993f935df85b970fa5d1b3ea42690 1
7960796-12740e69b63-b136e347d4df51a64c12dce88c0bfa2c 1
54345434-1237ddc95d7-5068e670f0a6e5973d021535d0a1bc2e 1
300a300a-123a68cec43-5068e670f0a6e5973d021535d0a1bc2e 1
29e629e6-126971c9d71-3f7993f935df85b970fa5d1b3ea42690 1
57585758-12aada2dfaf-b136e347d4df51a64c12dce88c0bfa2c 1
4a3c4a3c-12718b59527-3f7993f935df85b970fa5d1b3ea42690 1
7960796-12740e6374a-b136e347d4df51a64c12dce88c0bfa2c 1
57585758-12aada23eeb-b136e347d4df51a64c12dce88c0bfa2c 1
300a300a-123a68c465e-5068e670f0a6e5973d021535d0a1bc2e 1可以看到 原本null的 记录里的num（num为count（cid））都会为1，而其实这些为1的计数里有本来为null的，那么如何让这些为cid对应的aid为null的num计数显示为0呢？