使用 Dapper ORM 检查记录是否存在

Question

使用 Dapper ORM 检查记录是否存在的最简单方法是什么？

我真的需要为我只想检查记录是否存在的查询定义 POCO 对象吗？

Answer 1

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

应该可以工作...

Answer 2

我认为这可能会减少一点开销，因为没有函数调用或数据类型转换：

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

Answer 3

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 4

您可以让查询返回布尔值：

    [Test]
    public void TestExists()
    {
        var sql = @"with data as
                    (
                        select 1 as 'Id'
                    )
                    select CASE WHEN EXISTS (SELECT Id FROM data WHERE Id = 1)
                           THEN 1 
                           ELSE 0
                      END AS result 
                    from data ";

        var result = _connection.Query<bool>(sql).FirstOrDefault();

        Assert.That(result, Is.True);
    }

Answer 5

conn.QuerySingleOrDefault<bool>("select top 1 1 from table where id=@id", new { id});

Answer 6

另一个选项将运行重复记录，即不查询表的 id

bool exists = connection.ExecuteScalar<int>(
    "select count(1) from Table where notanId=@value", new { value = val})
     > 0;

Answer 7

如果您需要对非唯一字段执行此类查询，您可以使用

HAVING

来处理大于 1 的计数。

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

Answer 8

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>

映射到引用类型并导致装箱。

Answer 9

我正在使用这个语法，它对我、对 postgres 都有效。

select count(*)>0 from table_name where gid=@value;

对于甲骨文

SELECT CASE WHEN (B.C = 0) THEN 0 WHEN (B.C > 0) THEN 1 END boolean
  FROM (select count(*) as c from table_name where gid=@value) B
  WHERE B.C > 0;