题目描述：

Write an SQL query to report the number of experiments done on each of the three platforms for each of the three given experiments. Notice that all the pairs of (platform, experiment) should be included in the output including the pairs with zero experiments.
Return the result table in any order.
The query result format is in the following example.
Example 1:

方法1:
主要思路：解题链接汇总

select 'Android' as platform, 'Reading' as experiment_name,ifnull(count(*),0) as num_experiments
from Experiments
where platform = 'Android' and experiment_name ='Reading'
union all select 'Android' as platform, 'Sports' as experiment_name,ifnull(count(*),0) as num_experiments
from Experiments
where platform = 'Android' and experiment_name ='Sports'
union all select 'Android' as platform, 'Programming' as experiment_name,ifnull(count(*),0) as num_experiments
from Experiments
where platform = 'Android' and experiment_name ='Programming'
union all select 'IOS' as platform, 'Reading' as experiment_name,ifnull(count(*),0) as num_experiments
from Experiments
where platform = 'IOS' and experiment_name ='Reading'
union all select 'IOS' as platform, 'Sports' as experiment_name,ifnull(count(*),0) as num_experiments
from Experiments
where platform = 'IOS' and experiment_name ='Sports'
union all select 'IOS' as platform, 'Programming' as experiment_name,ifnull(count(*),0) as num_experiments
from Experiments
where platform = 'IOS' and experiment_name ='Programming'
union all select 'Web' as platform, 'Reading' as experiment_name,ifnull(count(*),0) as num_experiments
from Experiments
where platform = 'Web' and experiment_name ='Reading'
union all select 'Web' as platform, 'Sports' as experiment_name,ifnull(count(*),0) as num_experiments
from Experiments
where platform = 'Web' and experiment_name ='Sports'
union all select 'Web' as platform, 'Programming' as experiment_name,ifnull(count(*),0) as num_experiments
from Experiments
where platform = 'Web' and experiment_name ='Programming'