Question

使用Dapper ORM检查记录是否存在的最简单方法是什么？

我是否真的需要为查询定义POCO对象，我只想检查记录是否存在？

Answer 1

int id = ...
var exists = conn.ExecuteScalar<bool>("select count(1) from Table where Id=@id", new {id});

应该有用......

Answer 2

我认为由于没有函数调用或数据类型转换，这可能会有更少的开销：

int id = ...
var exists = connection.Query<object>(
    "SELECT 1 WHERE EXISTS (SELECT 1 FROM MyTable WHERE ID = @id)", new { id })
    .Any();

Answer 3

您可以让您的查询返回bool：

{{1}}

Answer 4

const string sql = "SELECT CAST(CASE WHEN EXISTS (SELECT 1 FROM MyTable WHERE Id = @Id) THEN 1 ELSE 0 END as BIT)";
bool exists = db.ExecuteScalar<bool>(sql, new { Id = 123 });

Answer 5

将使用重复记录运行的另一个选项，即不查询表的id

public void RemoveItem(int i) 
{
   // Removes rows with index i 
   myBindingList.RemoveAt(i)
}

Answer 6

如果您需要针对非唯一字段执行此类查询，则可以使用HAVING处理大于1的计数。

SELECT 1
FROM Table
WHERE Col=@val
HAVING COUNT(1) > 0

Answer 7

恕我直言SELECT TOP(1)比SELECT COUNT(1)更好

    bool exists = connection.Query<ValueTuple<long>>(
        "SELECT top(1) Id FROM MYTABLE WHERE MYTABLE.Id=@Id",
        new {Id}).Any());

ValueTuple<long>是值类型。 Query<object>映射到引用类型并引起装箱。

Answer 8

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使用Dapper ORM检查记录是否存在

8 个答案: