Question

我有2个表，需要加入它们才能获得所需的结果。我尝试了多种类型的连接，但没有运气。请帮忙。我的表格如下：

从t1中选择*

public static class QueryableUtils
{
    static Expression<Func<T, TResult>> Expr<T, TResult>(Expression<Func<T, TResult>> source) { return source; }

    static MethodInfo GetMethod(this LambdaExpression source) { return ((MethodCallExpression)source.Body).Method; }

    static readonly MethodInfo Object_ToString = Expr((object x) => x.ToString()).GetMethod();

    static readonly MethodInfo String_Contains = Expr((string x) => x.Contains("y")).GetMethod();

    public static IQueryable<T> Filter<T>(this IQueryable<T> query, List<SearchFilterDto> filters)
        // where T : BaseEntity
    {
        if (filters != null && filters.Count > 0 && !filters.Any(f => string.IsNullOrEmpty(f.Filter)))
        {
            var item = Expression.Parameter(query.ElementType, "item");
            var body = filters.Select(f =>
            {
                // Process the member path and build the final value selector
                Expression value = item;
                foreach (var memberName in f.Column.Split('.'))
                {
                    var member = item.Type.GetProperty(memberName, BindingFlags.IgnoreCase | BindingFlags.Public | BindingFlags.Instance) ??
                        (MemberInfo)item.Type.GetField(memberName, BindingFlags.IgnoreCase | BindingFlags.Public | BindingFlags.Instance);
                    if (member == null) return null; // Should probably throw an error?
                    value = Expression.MakeMemberAccess(value, member);
                }
                // NOTE: "Safe" skipping invalid arguments is not a good practice.
                // Without that requirement, the above block will be simply
                // var value = f.Column.Split('.').Aggregate((Expression)item, Expression.PropertyOrField);
                // Convert value to string if needed
                if (value.Type != typeof(string))
                {
                    // Here you can use different conversions based on the value.Type
                    // I'll just use object.ToString()
                    value = Expression.Call(value, Object_ToString);
                }
                // Finally build and return a call to string.Contains method
                return (Expression)Expression.Call(value, String_Contains, Expression.Constant(f.Filter));
            })
            .Where(r => r != null)
            .Aggregate(Expression.AndAlso);

            var predicate = Expression.Lambda<Func<T, bool>>(body, item);
            query = query.Where(predicate);
        }
        return query;
    }
}

从t2中选择*

ProductId, Sequence, Property
100, 1, Size
100, 2, Folder
100, 3, License
101, 1, Usage
101, 2, Duration

我需要加入他们才能获得以下结果：

SrNo, ProductId, Property, PropertyValue
1,    100, Size, 10GB
2,    100, Folder, /home/path
3,    101, Usage, Database

以下是重现相同表格的SQL：

SrNo, ProductId, Sequence, Property, PropertyValue
1     100         1         Size      10GB
1     100         2         Folder    
1     100         3         License     
2     100         1         Size      
2     100         2         Folder    /home/path
2     100         3         License   
3     101         1         Usage      Database
3     101         2         Duration

请协助我如何撰写查询？

Answer 1

SELECT
    table2.SrNo,
    table1.ProductId,
    table1.Sequence, 
    table1.Property,
    table2.PropertyValue
FROM t1 AS table1  
JOIN t2 AS table2 ON table1.ProductId = table2.ProductId

Answer 2

你去吧

OUTER JOINS

＆＃34;问题＆＃34;这是你将T2视为两个不同的表格。当我第一次从T2中选择时，我只选择SrNo和ProductId来获取这些＆＃34;有效＆＃34;的列表。值。然后我加入T1。然后我加入T2，将其视为PropertyValue的查找表。这里我使用左连接，因为并非所有Property和Sequence的组合都有一个有效的属性。

我＆＃34;修复＆＃34;上面的代码基于评论，但问题变得清晰。在您的示例中显示

SELECT T2.SrNo, T2.ProductId, T1.Sequence, T1.Property, T3.PropertyValue
FROM T2
JOIN T1 ON T2.ProductID = T1.ProductID
LEFT JOIN T2 AS T3 ON T1.Property = T3.Property

我不知道为什么第二行没有10GB的PropertyValue

SQL Join查询连接2个表以获得所需的输出

2 个答案: