如何以编程方式确定 SQL Server CE 数据库中是否存在表？

Question

当我的.sdf 文件中只有一个表时，这段代码运行良好：

const string sdfPath = @"\Program Files\duckbilled\Platypus.sdf";
string dataSource = string.Format("Data Source={0}", sdfPath);

if (!File.Exists(sdfPath))
{
    using (var engine = new SqlCeEngine(dataSource))
    {
        engine.CreateDatabase();
    }
    using (var connection = new SqlCeConnection(dataSource))
    {
        connection.Open();
        using (var command = new SqlCeCommand())
        {
            command.Connection = connection;
            command.CommandText =
                "CREATE TABLE Platydudes (Id int NOT NULL, BillSize smallint NOT NULL, Description nvarchar(255)";
            command.ExecuteNonQuery();
        }
    }
}

...但现在我需要知道的不是数据库文件（Platypus.sdf）是否存在，而是该表/文件中是否存在特定的表（例如Platydudes） .有没有办法确定？

更新

查询中的“IF NOT EXISTS”子句会导致运行时异常。这段代码：

using (var connection = new SqlCeConnection(dataSource))
{
    connection.Open();
    using (var command = new SqlCeCommand())
    {
        command.Connection = connection;
        command.CommandText = "IF NOT EXISTS( SELECT 1 FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME = 'InventoryItems') " +
            "CREATE TABLE InventoryItems (Id nvarchar(50) NOT NULL, PackSize smallint NOT NULL, Description nvarchar(255), DeptDotSubdept numeric, UnitCost numeric, UnitList numeric, UPCCode nvarchar(50), UPCPackSize smallint, CRVId int);";
        command.ExecuteNonQuery();
    }
}

...导致抛出此异常：解析查询时出错。 [令牌行号=1，令牌行偏移量=1，错误令牌=IF]

因此，查询解析器显然不需要“IF”业务。如果表不存在，是否有另一种方法只创建表？还是我应该每次都先删除表然后重新创建它？ IOW，我应该这样做吗：

using (var connection = new SqlCeConnection(dataSource))
{
    connection.Open();
    using (var command = new SqlCeCommand())
    {
        command.Connection = connection;
        command.CommandText = "DELETE InventoryItems";
        command.ExecuteNonQuery();
    }
    using (var command = new SqlCeCommand())
    {
        command.Connection = connection;
        command.CommandText = "CREATE TABLE InventoryItems (Id nvarchar(50) NOT NULL, PackSize smallint NOT NULL, Description nvarchar(255), DeptDotSubdept numeric, UnitCost numeric, UnitList numeric, UPCCode nvarchar(50), UPCPackSize smallint, CRVId int);";
        command.ExecuteNonQuery();
    }
}

?

更新 2

在第一次更新中回答我上面的问题：不！如果我这样做，我会在第二次调用 .ExecuteNonQuery() 时得到“指定的表已经存在”。

更新 3

回应 Shiva 对我的回答的评论：

这（重用命令对象）以同样的方式失败（“表已经存在”）：

using (var command = new SqlCeCommand())
{
    command.Connection = connection;
    command.CommandText = "DELETE InventoryItems";
    command.ExecuteNonQuery();
    command.CommandText = "CREATE TABLE InventoryItems (Id nvarchar(50) NOT NULL, PackSize smallint NOT NULL, Description nvarchar(255), DeptDotSubdept numeric, UnitCost numeric, UnitList numeric, UPCCode nvarchar(50), UPCPackSize smallint, CRVId int);";
    command.ExecuteNonQuery();
}

Answer 1

SQL Compact 不太喜欢 SQL 语句中的逻辑，因此 Shiva 的回答可能不会通过解析器。不过，他走在正确的道路上。您可以分两步完成：

这是OpenNETCF ORM 的 SQL Compact 实现中的 TableExists 方法：

public override bool TableExists(string tableName)
{
    var connection = GetConnection(true);
    try
    {
        using (var command = GetNewCommandObject())
        {
            command.Transaction = CurrentTransaction as SqlCeTransaction;
            command.Connection = connection;
            var sql = string.Format(
                    "SELECT COUNT(*) FROM information_schema.tables WHERE table_name = '{0}'", 
                     tableName);
            command.CommandText = sql;
            var count = Convert.ToInt32(command.ExecuteScalar());

            return (count > 0);
        }
    }
    finally
    {
        DoneWithConnection(connection, true);
    }
}

显然，对于您的情况，它并不完整，但它很容易理解。 CurrentTransaction 很容易为空。 GetNewCommandObject 只返回一个新的 SqlCeCommand 实例。 GetConnection 可以简单地返回一个新的 SqlCeConnection 实例。 DoneWithConnection 可能是一个 NOP。基本上这些都是处理 ORM 支持过多的后备存储的事实。您需要的核心信息是我传入的 SQL 以及我如何确定真假返回。

我（未经编译器测试）对结果方法的猜测是这样的：

public bool TableExists(SqlCeConnection connection, string tableName)
{
    using (var command = new SqlCeCommand())
    {
        command.Connection = connection;
        var sql = string.Format(
                "SELECT COUNT(*) FROM information_schema.tables WHERE table_name = '{0}'", 
                 tableName);
        command.CommandText = sql;
        var count = Convert.ToInt32(command.ExecuteScalar());
        return (count > 0);
    }
}

Answer 2

如果你想检查一个表是否存在，你必须使用TABLE_SCHEMA

IF (EXISTS (SELECT * 
             FROM INFORMATION_SCHEMA.TABLES 
             WHERE TABLE_SCHEMA = 'TheSchema' 
             AND  TABLE_NAME = 'TheTable'))
BEGIN
--Do Stuff
END

Answer 3

使用数据库助手：

var db = Database.Open("MyDatabase");
var sql = @"SELECT Count(*) FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME = 'MyTable'"
var count = db.QueryValue(sql);
if(count.Equals(1)){
    //table exists
}

Answer 4

根据 Shiva 的回答，我会这样做：

if (!File.Exists(sdfPath))
{
    using (var engine = new SqlCeEngine(dataSource))
    {
        engine.CreateDatabase();
    }
}

using (var connection = new SqlCeConnection(dataSource))
{
    connection.Open();
    using (var command = new SqlCeCommand())
    {
        command.Connection = connection;
        command.CommandText ="IF NOT EXISTS( SELECT 1 FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME = 'Platydudes') " 
                           + "CREATE TABLE Platydudes (Id int NOT NULL, BillSize smallint NOT NULL, Description nvarchar(255) )";
        command.ExecuteNonQuery();
    }
}

...根据需要为每个表（Platypups、Platypops 等）重复“使用（var connection = ...”部分）

更新

使用 ctacke 的代码，我现在有了这个：

public static void ConditionallyCreateTables()
{
    const string sdfPath = @"\Program Files\duckbilled\Platypus.sdf";
    string dataSource = string.Format("Data Source={0}", sdfPath);

    if (!File.Exists(sdfPath))
    {
        using (var engine = new SqlCeEngine(dataSource))
        {
            engine.CreateDatabase();
        }
    }
    using (var connection = new SqlCeConnection(dataSource))
    {
        connection.Open();
        using (var command = new SqlCeCommand())
        {
            command.Connection = connection;

            if (!TableExists(connection, "InventoryItems"))
            {
                command.CommandText = "CREATE TABLE InventoryItems (Id nvarchar(50) NOT NULL, PackSize smallint NOT NULL, Description nvarchar(255), DeptDotSubdept numeric, UnitCost numeric, UnitList numeric, UPCCode nvarchar(50), UPCPackSize smallint, CRVId int);";
                command.ExecuteNonQuery();
            }

            if (!TableExists(connection, "Departments"))
            {
                command.CommandText = "CREATE TABLE Departments (Id int NOT NULL, DeptNum int NOT NULL, DepartmentName nvarchar(255))";
                command.ExecuteNonQuery();
            }

            if (!TableExists(connection, "Subdepartments"))
            {
                command.CommandText = "CREATE TABLE Subdepartments (Id int NOT NULL, DeptId int NOT NULL, SubdeptId int NOT NULL, DepartmentName nvarchar(255))";
                command.ExecuteNonQuery();
            }

            if (!TableExists(connection, "Redemptions"))
            {
                command.CommandText = "CREATE TABLE Redemptions (Id int NOT NULL, RedemptionId nvarchar(50), RedemptionItemId nvarchar(50), RedemptionName nvarchar(255), RedemptionAmount numeric, RedemptionDept nvarchar(50), RedemptionSubdept nvarchar(50))";
                command.ExecuteNonQuery();
            }
        }
    }
}

更新 2

我现在已将其更改为这种格式：

if (TableExists(connection, "InventoryItems"))
{
    command.CommandText = "DROP TABLE InventoryItems";
    command.ExecuteNonQuery();
}
command.CommandText = "CREATE TABLE InventoryItems (Id nvarchar(50) NOT NULL, PackSize smallint NOT NULL, Description nvarchar(255), DeptDotSubdept float, UnitCost float, UnitList float, UPCCode nvarchar(50), UPCPackSize smallint, CRVId int);";
command.ExecuteNonQuery();