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Entity Framework常用查询，EF join,EF多表联查
于 2021-09-12 22:19:58 发布
cxnwb
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收藏 11
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文章标签： 数据库开发
Ef 两表Join
linq写法
1
//两表join linq写法
2
var query = from u in oae.Users
3
join p in oae.Parent on u.Id equals p.ParentId
4
select new
5
{
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username = u.UserName,
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father = p.Father
8
};
lamdba写法：
1
/*
2
第一个参数：
3
第二，三参数： 连接条件
4
5
第四个参数：
join的表
返回值
*/
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var query = oae.Users.Join(oae.Parent, a => a.Id, b => b.ParentId, (a, b) => new
7
{ 8
9
10
username = a.UserName,
fahter = b.Father
});
Ef 两表 left Join
linq写法：
1
//两表left join linq写法
2
var query = from u in oae.Users
3
join p in oae.Parent on u.Id equals p.ParentId into jtemp
4
from leftjoin in jtemp.DefaultIfEmpty()
5
select new
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{
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username = u.UserName,
8
father = leftjoin.Father
9
};
lamdba写法：
1
//两表left join lamdba写法
2
var query = oae.Users.GroupJoin(oae.Parent, a => a.Id, b => b.ParentId, (a, b) => new
3
{ 4
5
username = a.UserName,
parent = b
6
}).SelectMany(a => a.parent, (m, n) => new
7
{
8
username = m.username,
9
10
father = n.Father
});
lamdba的写法主要用到了groupjoin与SelectMany,这里简单解释一下：
groupjoin：
用于查询一对多的关系很方便，所以得数据格式就是1对多的关系
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SelectMany:
可以解析集合中含有集合的情况(也就是1对多的表现)为单一对象
Ef三表Join
linq写法：
1
//三表join linq写法
2
var queru = from u in oae.Users
3
join p in oae.Parent on u.Id equals p.ParentId
4
join s in oae.Score on u.Id equals s.UsersId
5
select new
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{
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username = u.UserName,
8
fahter = p.Father,
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sub = s.Sub,
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score = s.Score1
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};
lamdba写法：
1
//三表join lamdba写法
2
var query = oae.Users.Join(oae.Parent, a => a.Id, b => b.ParentId, (a, b) => new
3
{
4
uid = a.Id,
5
username = a.UserName,
6
father = b.Father
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}).Join(oae.Score, a => a.uid, b => b.UsersId, (m, n) => new
8
{
9
username = m.username,
10
father = m.father,
11
sub = n.Sub,
12
score = n.Score1
13
});
其实和两表join类似，往后面点就行了
Ef三表left Join
Linq写法：
1
//三表left join linq写法
2
var query = from u in oae.Users
3
join p in oae.Parent on u.Id equals p.ParentId into ptemp
4
join s in oae.Score on u.Id equals s.UsersId into stemp
5
from leftp in ptemp.DefaultIfEmpty()
6
from lefts in stemp.DefaultIfEmpty()
7
select new
8
{
9
username = u.UserName,
10
father = leftp.Father,
11
sub = lefts.Sub,
12
score = lefts.Score1
13
};
lamdba写法：
1
//三表left join lamdba写法
2
var query = oae.Users.GroupJoin(oae.Parent, a => a.Id, b => b.ParentId, (a, b) => new
3
{ 4
5
uid = a.Id,
username = a.UserName,
6
parent = b
7
}).GroupJoin(oae.Score, a => a.uid, b => b.UsersId, (m, n) => new
8
{
9
10
username = m.username,
uid = m.uid,
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11
score = n,
12
parent = m.parent
13
}).SelectMany(a => a.parent.DefaultIfEmpty(), (m, n) => new
14
{
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username = m.username,
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fahter = n.Father,
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score = m.score
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}).SelectMany(a => a.score.DefaultIfEmpty(), (m, n) => new
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{
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usernaem = m.username,
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father = m.fahter,
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sub = n.Sub,
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score = n.Score1
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});
lamdba写法2：上面是现join完在selectmany，也可以先selectmany了在join第三张表
1
//三表left join lamdba写法2
2
var query = oae.Users.GroupJoin(oae.Parent, a => a.Id, b => b.ParentId, (a, b) => new
3
{ 4
5
uid = a.Id,
username = a.UserName,
6
parent = b
7
}).SelectMany(a => a.parent.DefaultIfEmpty(), (m, n) => new
8
{
9
uid = m.uid,
10
username = m.username,
11
father = n.Father
12
}).GroupJoin(oae.Score, a => a.uid, b => b.UsersId, (m, n) => new
13
{
14
username = m.username,
15
father = m.father,
16
score = n
17
}).SelectMany(a => a.score, (m, n) => new
18
{
19
username = m.username,
20
father = m.father,
21
sub = n.Sub,
22
score = n.Score1
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});
单表分组函数
linq:
1
//linq
2
var query = from score in oae.Score
3
group score by score.Sub into grouptemp
4
select new
5
{
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sub = grouptemp.Key,
7
sum = grouptemp.Sum(a => a.Score1),
8
max = grouptemp.Max(a => a.Score1),
9
min = grouptemp.Min(a => a.Score1),
10
avg = grouptemp.Average(a => a.Score1)
11
};
lamdba:
1
//lamdba
2
var query = oae.Score.GroupBy(a => a.Sub).Select(grouptemp => new
3
{
4
sub = grouptemp.Key,
5
sum = grouptemp.Sum(a => a.Score1),
6
max = grouptemp.Max(a => a.Score1),
7
min = grouptemp.Min(a => a.Score1),
8
9
avg = grouptemp.Average(a => a.Score1)
}).Where(a => a.max > 60);
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10
11
var result = query.ToList();
分组函数后接一点条件
linq:
1
//linq
2
var query = from score in oae.Score
3
group score by score.Sub into grouptemp
4
where grouptemp.Sum(a=>a.Score1)>60
5
select new
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{
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sub = grouptemp.Key,
8
sum = grouptemp.Sum(a => a.Score1),
9
max = grouptemp.Max(a => a.Score1),
10
min = grouptemp.Min(a => a.Score1),
11
avg = grouptemp.Average(a => a.Score1)
12
};
linq写法2：
1
//linq
2
var query = from score in oae.Score
3
group score by score.Sub into grouptemp
4
select new
5
{
6
sub = grouptemp.Key,
7
sum = grouptemp.Sum(a => a.Score1),
8
max = grouptemp.Max(a => a.Score1),
9
min = grouptemp.Min(a => a.Score1),
10
avg = grouptemp.Average(a => a.Score1)
11
} into temp
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where temp.max > 60
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select new
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{
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sub = temp.sub,
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sum = temp.sum
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};
var result = query.ToList();
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