文章目录
记录1
背景
表1：problems（习题表，存放习题的基本信息）
表2：submissions（提交记录表，用户对习题的提交记录，包含提交内容、提交结果、时间等信息）
开发语言是Go，使用的ORM框架是Gorm
需求1：按题目分组查询用户提交数
SQL
SELECT
problem,count(*)
FROM
submissions
WHERE
creator = 5
GROUP BY problem
Gorm写法
func test(user int) {
conn := db.GetDBConnection()
conn.Model(model.Submission{
}).Select("problem, count(*) submission_count").
Where("a.creator=?", user)
}
需求2：按题目分组查询，并统计count，获取最新的时间
SQL
SELECT
max(a.created_at) last_time,
b.title,
b.difficulty,
count(*) submission_count
FROM
submissions a
LEFT JOIN problems b ON a.problem = b.id
WHERE
a.creator = 5
GROUP BY
a.problem
Gorm写法
func test(user int) {
conn := db.GetDBConnection()
conn.Table("(?) a", conn.Model(model.Submission{
})).
Joins("left join problems b on a.problem=b.id").
Select("max(a.created_at) last_time,b.title,b.difficulty, count(*) submission_count").
Where("a.creator=?", user).Group("a.problem")
}
需求3：在需求2的基础上，按时间倒序排序
分析：
需要按题目分组的同时，获取到最后一次提交的时间，并按提交的先后顺序倒叙排序，所以做两次分组：
第一次用题目ID和主键ID分组，并按主键倒叙排序；（主键为自增）
第二次使用第一次的查询作为子查询，按题目ID分组，使用any_value()函数获取同一分组中的第一条记录的时间，也就是最近一次提交的时间，
需求2中，直接使用max(created_at)获取最新的提交时间，只使用题目ID分组，无法满足分组的同时，实现所有记录的倒叙排序
SQL
SELECT
t.problem,
any_value ( t.created_at ) last_time,
t.title,
t.difficulty
FROM
(
SELECT
submissions.problem,
submissions.created_at,
problems.title,
problems.difficulty
FROM
`submissions`
LEFT JOIN problems ON submissions.problem = problems.id
WHERE
( submissions.creator = 5 )
AND `submissions`.`deleted_at` IS NULL
GROUP BY
submissions.problem,
submissions.id
ORDER BY
submissions.id DESC
) AS t
GROUP BY
`t`.`problem`
Gorm写法
type MySubmissionsDTO struct {
Title string `json:"title" form:"title"` // 标题
Difficulty string `json:"difficulty" form:"difficulty"` // 难度
LastTime time.Time `json:"last_time" form:"last_time"` // 最后提交时间
PassedTime string `json:"passed_time" form:"passed_time"` // 经过的时间
ProblemSubmissionCount
}
type ProblemSubmissionCount struct {
Problem uint `json:"problem" form:"problem"` // 题目ID
SubmissionCount int `json:"submission_count" form:"submission_count"` // 提交次数
}
func TestSql(user uint) {
conn := db.GetDBConnection()
tx := conn.Model(&model.Submission{
}).
Select("submissions.problem,submissions.created_at,problems.title,problems.difficulty").
Joins("left join problems on submissions.problem = problems.id").
Where("submissions.creator=?", user)
//第一次分组：根据题目ID+主键ID分组
tx.Group("submissions.problem,submissions.id")
//按主键ID倒叙排序
tx.Order("submissions.id desc")
//第二次分组：根据题目ID分组（from子查询示例：子查询的参数是一个*gorm.DB对象）
tx2 := conn.Table("(?) as t", tx).Select("t.problem,any_value(t.created_at) last_time,t.title,t.difficulty").
Group("t.problem")
//计算count时，最好是将子查询包起来，避免count值有误
var total int64
conn.Table("(?) as t1", tx2).Count(&total)
var list [] MySubmissionsDTO
tx2.Find(&list)
fmt.Println("total: ", total)
fmt.Println("list: ", list)
}
需求4：同时查询多个不同条件count值
SQL
select
(SELECT
count(*)
FROM
submissions
WHERE
creator = 5
AND result = 1) passed_count,
(SELECT
count(*)
FROM
submissions
WHERE
creator = 5 ) submission_count
或：
SELECT
*
FROM
( SELECT count(*) submission_count FROM `submissions` WHERE ( submissions.creator = 5 ) AND `submissions`.`deleted_at` IS NULL ) AS submission_count,
( SELECT count(*) passed_count FROM `submissions` WHERE ( submissions.creator = 5 AND result = 1 ) AND `submissions`.`deleted_at` IS NULL ) AS passed_count
Gorm写法
type Result struct {
PassedCount int `json:"passed_count" form:"passed_count"` // 已通过题目数量
SubmissionTotalCount int `json:"submission_total_count" form:"submission_total_count"` // 提交总数
}
func test() {
conn := db.GetDBConnection()
submissionCountSql := conn.Model(&model.Submission{
}).Select("count(*) submission_total_count").
Where("submissions.creator=?", 5)
passedCountSql := conn.Model(&model.Submission{
}).Select("count(*) passed_count").
Where("submissions.creator=? and result=1", 5)
var result = Result{
}
conn.Table("(?) as submission_total_count,(?) as passed_count", submissionCountSql, passedCountSql).
Find(&result)
fmt.Println("result: ", result)
}