I want to read data from a table whose name is supplied by a user. So before actually starting to read data, I want to check if the database exists or not.
I have seen several pieces of code on the NET which claim to do this. However, they all seem to be work only for SQL server, or for mysql, or some other implementation. Is there not a generic way to do this?
(I am already seperately checking if I can connect to the supplied database, so I'm fairly certain that a connection can be opened to the database.)
You cannot do this in a cross-database way. Generally DDL (that is, the code for creating tables, indexes and so on) is completely different from database to database and so the logic for checking whether tables exist is also different.
I would say the simplest answer, though, would simply be something like:
SELECT * FROM <table> WHERE 1 = 0
If that query gives an error, then the table doesn't exist. If it works (though it'll return 0 rows) then the table exists.
Be very careful with what you let the user input, though. What's to stop him from from specifying "sysusers" as the table name (in SQL Server, that'll be the list of all database users)
You can use the DbConnection.GetSchema family of methods to retreive metadata about the database. It will return a DataTable with schema objects. The exact object types and restriction values may vary from vendor to vendor, but I'm sure you can set up your check for a specific table in a way that will work in most databases.
Here's an example of using GetSchema that will print the name and owner of every table that is owned by "schema name" and called "table name". This is tested against oracle.
static void Main(string[] args)
{
string providerName = #"System.Data.OracleClient";
string connectionString = #"...";
DbProviderFactory factory = DbProviderFactories.GetFactory(providerName);
using (DbConnection connection = factory.CreateConnection())
{
connection.ConnectionString = connectionString;
connection.Open();
DataTable schemaDataTable = connection.GetSchema("Tables", new string[] { "schema name", "table name" });
foreach (DataColumn column in schemaDataTable.Columns)
{
Console.Write(column.ColumnName + "\t");
}
Console.WriteLine();
foreach (DataRow row in schemaDataTable.Rows)
{
foreach (object value in row.ItemArray)
{
Console.Write(value.ToString() + "\t");
}
Console.WriteLine();
}
}
}
That's like asking "is there a generic way to get related data" in databases. The answer is of course no - the only "generic way" is to have a data layer that hides the implementation details of your particular data source and queries it appropriately.
If you are really supporting and accessing many different types of databases without a Stategy design pattern or similar approach I would be quite surprised.
That being said, the best approach is something like this bit of code:
bool exists;
try
{
// ANSI SQL way. Works in PostgreSQL, MSSQL, MySQL.
var cmd = new OdbcCommand(
"select case when exists((select * from information_schema.tables where table_name = '" + tableName + "')) then 1 else 0 end");
exists = (int)cmd.ExecuteScalar() == 1;
}
catch
{
try
{
// Other RDBMS. Graceful degradation
exists = true;
var cmdOthers = new OdbcCommand("select 1 from " + tableName + " where 1 = 0");
cmdOthers.ExecuteNonQuery();
}
catch
{
exists = false;
}
}
Source: Check if a SQL table exists
You can do something like this:
string strCheck = "SHOW TABLES LIKE \'tableName\'";
cmd = new MySqlCommand(strCheck, connection);
if (connection.State == ConnectionState.Closed)
{
connection.Open();
}
cmd.Prepare();
var reader = cmd.ExecuteReader();
if (reader.HasRows)
{
Console.WriteLine("Table Exist!");
}
else (reader.HasRows)
{
Console.WriteLine("Table Exist!");
}
Related
Having a little bit of a strange error here that I have never encountered before. I have an application where users can type in a list of accounts in a datagrid and a date range and press a button and it will return the data for these accounts in a datagrid and give them the option to export it to an excel file. This works perfectly for me, logged in under my username and even when I log in under other people's username. The problem is when they try it, they get no data back. No errors, just it doesn't pull any data.
The interesting thing is this is all in the same database as the other information which they access without any problem. The only difference, which I think might be the explanation is I am calling this SQL code directly from the Application whereas everything else is called using stored procedures that sit on the server. The reason for this is I have to concatenate the SQL Query string for each item in the accounts field. Since they are able to enter as many accounts as they want, I cannot use a stored procedure since I don't know how many parameters it will have ultimately(if someone could let me know a method of doing this, I would actually prefer this way for keeping things consistent). Obviously the query string is working properly, as it's pulling data back for me, but the question I have is why is it failing to return data for others? The connection string is an SQL Authentication, so it shouldn't have anything to do with them not having Windows Authentication on the server, plus they are already able to log in to the application and it displays data on their dashboard, which couldn't happen...
Anyone that can point me in the right direction with this I would appreciate it...the only thing I can think of is it is an issue with using an in-code SQL string versus a stored procedure, but this doesn't make any sense since other people do this all the time in applications without issue.
public ICommand GetData
{
get => new RelayCommand(() =>
{
//call the SQL Code to lookup the account numbers
var SQLStr = "SELECT * FROM [Clients].[Data] WHERE (Account_Number = '";
for (var i = 0; i< AccountNums.Count; i++)
{
if (!String.IsNullOrEmpty(AccountNums[i].accNum)) SQLStr += i == 0 ? $"{AccountNums[i].accNum}'" : $" OR Account_Number = '{AccountNums[i].accNum}'";
}
SQLStr += $") AND SUB_QUERY_CREATED_ON BETWEEN '{StartDate.ToString()}' AND '{EndDate.ToString()}'";
_Data = DBMethods.GetSQLData(_Data, new Models.Clients.Data(), SQLStr, new List<string> { "ID" }, true);
ShowResPnl = true; //there are results, toggle the panel visibility bound variable
});
}
public static ObservableCollection<T> GetSQLData<T>(ObservableCollection<T> myCollection, T myClass, String SQLString, List<string> remParams, bool UseSQLQuery) where T : class
{
var conn = new SqlConnection();
try
{
var paramList = GenerateSQLParameters(myClass, remParams);
using (getConnection(conn))
{
conn.Open();
using (SqlCommand cmd = new SqlCommand(SQLString, conn))
{
cmd.CommandType = CommandType.Text;
SqlDataReader reader;
reader = cmd.ExecuteReader();
//only execute if the reader has data
if (reader.HasRows)
{
while (reader.Read())
{
var tempModel = Global.GenerateNewInstance(myClass) as T;
Type model = tempModel.GetType();
var prop = model.GetProperties();
PropertyInfo pi;
//set the values for each property in the model
foreach (var p in prop)
{
if (!remParams.Contains(p.Name))
{
pi = tempModel.GetType().GetProperty(p.Name);
if (reader[p.Name] == DBNull.Value)
{
pi.SetValue(tempModel, null);
}
else
{
pi.SetValue(tempModel, reader[p.Name]);
}
}
}
myCollection.Add(tempModel);
}
reader.Close();
cmd.Dispose();
}
}
}
}
catch (Exception ex)
{
ErrorWindow errWin = new ErrorWindow("There was a problem trying to Get the Data with the Query '" + SQLString + "'! Error: " + ex.Message);
errWin.Show();
}
return myCollection;
}
UPDATE: OK I got it working perfectly with help from THIS thread:
How do I split a string so I can access item x?
and more specifically this post:
What about using string and values() statement?
DECLARE #str varchar(max)
SET #str = 'Hello John Smith'
DECLARE #separator varchar(max)
SET #separator = ' '
DECLARE #Splited TABLE(id int IDENTITY(1,1), item varchar(max))
SET #str = REPLACE(#str, #separator, '''),(''')
SET #str = 'SELECT * FROM (VALUES(''' + #str + ''')) AS V(A)'
INSERT INTO #Splited
EXEC(#str)
SELECT * FROM #Splited
I created a stored procedure using this, then did a left join on Account numbers from the Data Table and used a WHERE clause to set the Start and End Dates and exclude items that were NULL(checked one of the columns). Works perfectly and only took about 2 or 3 seconds to return the data. I had another working method as detailed here https://sqlperformance.com/2012/07/t-sql-queries/split-strings#comments using a function which was taking well over a minute to return data for only 4 accounts...obviously was not going to work well enough so I found the method mentioned prior and it works excellently!
I've been tasked with creating a backup of the data in our "default schema" database dbo to the same database using a new schema called dbobackup.
I honestly do not understand what this means as far as a database goes. Apparently, it is like having a database backup inside the existing database. I guess there is some advantage to doing that.
Anyway, I can't seem to find anywhere online that will allow me to do this.
I have found a few posts on here about copying the schema without data, but I need the data too.
Backup SQL Schema Only?
How do I check to see if a schema exists, delete it if it does, and then create a schema that accepts data in the current database?
Once I have the new schema created, can I dump data in there with a simple command like this?
SELECT * INTO [dbobackup].Table1 FROM [dbo].Table1;
That line only backs up one table, though. If I need to do this to 245 tables for this particular customer, I'd need a script.
We have several customers, too, and their databases are not structured identically.
Could I do something along these lines?
I was thinking about creating a small console program to walk through the tables.
How would I modify something like the code below to do what I want?
public static void Backup(string sqlConnection)
{
using (var conn = new SqlConnection(sqlConnection))
{
conn.Open();
var tables = new List<String>();
var sqlSelectTables = "SELECT TableName FROM [dbo];";
using (var cmd = new SqlCommand(sqlSelectTables, conn))
{
using (var r = cmd.ExecuteReader())
{
while (r.Read())
{
var item = String.Format("{0}", r["TableName"]).Trim();
tables.Add(item);
}
}
}
var fmtSelectInto = "SELECT * INTO [dbobackup].{0} FROM [dbo].{0}; ";
using (var cmd = new SqlCommand(null, conn))
{
foreach (var item in tables)
{
cmd.CommandText = String.Format(fmtSelectInto, item);
cmd.ExecuteNonQuery();
}
}
}
}
SQL Server already has this built in. If you open SQL Server Management Studio and right click on the database you want to back up, then select all tasks then backup, you will get an option to back up your database into an existing database.
This is the important part and why you should use the built in functionality: You must copy the data from one DB to the other DB in the correct order or you'll get foreign key errors all over the place. If you have a lot of data tables with a lot of relationships, this will really be hard to nail down on your own. You could write code to make a complete graph of all of the dependencies and then figure out what order to copy the table data (which is essentially what SQL Server already does).
Additionally, there are third-party programs available to do this type of backup as well (see: Google).
This is sort of a "work in progress" approach I got started with that looks promising:
public static void CopyTable(
string databaseName, // i.e. Northwind
string tableName, // i.e. Employees
string schema1, // i.e. dbo
string schema2, // i.e. dboarchive
SqlConnection sqlConn)
{
var conn = new Microsoft.SqlServer.Management.Common.ServerConnection(sqlConn);
var server = new Microsoft.SqlServer.Management.Smo.Server(conn);
var db = new Microsoft.SqlServer.Management.Smo.Database(server, databaseName);
db.Tables.Refresh();
for (var itemId = 0; itemId < db.Tables.Count; itemId++)
{
var table = db.Tables.ItemById(itemId);
if (table.Name == tableName)
{
table.Schema = String.Format("{0}", DatabaseSchema.dboarchive);
table.Create();
}
}
}
The only issue I am currently running into is that my db variable always comes back with Tables.Count == 0.
If I get a chance to fix this, I will update.
For now, I've been told to remove this piece of code and check my code in.
All right, this is the bigger question linked to this link which I tried to delete but couldnt any more. People said I should never post part of the problem due to the x y problem link, which is fair enough. So here it comes.
Lets say I have a class:
public class CustomClass
{
public string Year;
public double val;
public string Tariff;
public string ReportingGroup;
}
I now have some process that creates a list of this class with results (in reality its a bigger class but that shouldnt matter).
I now create an Access table if it doesnt exist yet. For this I need the class members and ideally also the type (currently all text!):
public static void createtable(string path, string tablename, string[] columnnames)
{
try
{
string connectionstring = creadteconnectionstring(path);
OleDbConnection myConnection = new OleDbConnection(connectionstring);
myConnection.Open();
OleDbCommand myCommand = new OleDbCommand();
myCommand.Connection = myConnection;
string columnam = "[" + columnnames[0] + "] Text";
for (int i = 1; i < columnnames.Length; i++)
{
columnam = columnam + ", [" + columnnames[i] + "] Text";
}
myCommand.CommandText = "CREATE TABLE [" + tablename + "](" + columnam + ")";
myCommand.ExecuteNonQuery();
myCommand.Connection.Close();
Console.WriteLine("Access table " + tablename + " created.");
}
catch
{
Console.WriteLine("Access table " + tablename + " already exists.");
return;
}
}
Note column name contains actually the names of the class members of custom class. Then I paste the data into it:
public static void appenddatatotable(string connectionstring, string tablename, string datstr, List<CustomClass> values)
{
string commandtext = "INSERT INTO " + tablename + " ([RunDate],[ReportingGroup], [Tariff], [Year], [Quarter]) VALUES(#RunDate, #ReportingGroup, #Tariff, #Year, #Quarter)";
using (var myconn = new OleDbConnection(connectionstring))
{
myconn.Open();
using (var cmd = new OleDbCommand())
{
foreach (var item in values)
{
cmd.CommandText = commandtext;
if (string.IsNullOrEmpty(item.val))
item.val = "";
cmd.Parameters.Clear();
cmd.Parameters.AddRange(new[] { new OleDbParameter("#RunDate", datstr), new OleDbParameter("#ReportingGroup", item.RG), new OleDbParameter("#Tariff", item.tar), new OleDbParameter("#Year", item.yr), new OleDbParameter("#Quarter", item.val)});
cmd.Connection = myconn;
//cmd.Prepare();
cmd.ExecuteNonQuery();
}
}
}
}
This all works fine.
However, say I change sth in my process that also needs another calculation that yields value2, then I need to change the class, the createtable and teh appenddatatotable function. I would like to only update the class.
So, you are trying to build your own ORM (Object Relational Mapper) for C# and MS Access databases.
While this is an interesting endeavour as a learning experience, it's a problem that is hard to tackle properly.
What you need to do is use reflection in your createtable to determine the details metadata necessary (property names, property types) to construct the CREATE TABLE SQL Statement.
Then you could use something like DBUtils.CreateTable<CustomClass>(connStr); to create the table.
Since you have not mentioned reflection in this question, you really need to first learn as much as you can about it, and experiment with it first before you can answer your own question.
You previous question had some answers that already mentioned using reflection and showed you how to get the property names and types of arbitrary classes.
Once you get through that hurdle, you will encounter other problems:
How to define type lengths
Especially for strings, in .Net they can be considered almost unlimited (for most use anyway) but in Access, a string of less than 255 characters is not the same type as a larger one.
How to define your Primary key.
As a general rule, all tables in a database must have a Primary Key field that is used to identify each record in a table in a unique way.
In an ORM, it's really important, so you can easily fetch data based on that key, like GetByID<CustomClass>(123) would return an instance of your CustomClass that contains the data from record whose primary key ID is 123.
How to define indexes in your database.
Creating tables is all good and well, but you must be able to define indexes so that queries will have expected performance.
How to define relationships between tables.
Databases are all about relational data, so you need a way to define these relationships within your classes so that a class PurchaseOrder can have a list of PurchaseItem and your code understand that relationship, for instance when you need to delete a given Purchase Order, you will also need to delete all of its items in the database.
How to only load what you need.
Say you have a Customer class that has a PurchaseOrders property that is in fact a List<PurchaseOrders>. Now, if you load the data of a particular customer, to display their phone number for instance, you do not want to also pull all the possible 1,000s or orders they have made over the years, each of these having maybe 100s of items...
How to execute queries and use their results.
Once you have mapped all your tables to classes, how do you query your data?
linq is fantastic, but it's very hard to implement by yourself, so you need a good solution to allow you to make queries and allow your queries to return typed data.
For many of these issues, custom Attributes are the way to go, but as you move along and make your ORM more powerful and flexible, it will increase in complexity, and your early decisions will sometimes weigh you down and complicate things further because, let's face it, building an ORM from scratch, while an interesting experience, is hard.
So, you really have to think about all these questions and set yourself some limits on what you need/want from the system before jumping into the rabbit hole.
Ok, so here's the problem I have to solve. I need to write a method in C# that will modify a table in SQL Server 2008. The table could potentially contain millions of records. The modifications include altering the table by adding a new column and then calculating and setting the value of the new field for every row in the table.
Adding the column is not a problem. It's setting the values efficiently that is the issue. I don't want to read in the whole table into a DataTable and then update and commit for obvious reasons. I'm thinking that I would like to use a cursor to iterate over the rows in the table and update them one by one. I haven't done a whole lot of ADO.NET development, but it is my understanding that only read-only server side (firehose) cursors are supported.
So what is the correct way to go about doing something like this (preferably with some sample code in C#)? Stored procedures or other such modifications to the DB are not allowed.
jpgoody,
Here is an example to chew on using the NerdDinner database and some SQLConnection, SQLCommand, and SQLDataReader objects. It adds one day to each of the Event Dates in the Dinners table.
using System;
using System.Data.SqlClient;
namespace NerdDinner
{
public class Class1
{
public void Execute()
{
SqlConnection readerConnection = new SqlConnection(Properties.Settings.Default.ConnectionString);
readerConnection.Open();
SqlCommand cmd = new SqlCommand("SELECT DinnerID, EventDate FROM Dinners", readerConnection);
SqlDataReader reader = cmd.ExecuteReader();
SqlConnection writerConnection = new SqlConnection(Properties.Settings.Default.ConnectionString);
writerConnection.Open();
SqlCommand writerCommand = new SqlCommand("", writerConnection);
while (reader.Read())
{
int DinnerID = reader.GetInt32(0);
DateTime EventDate = reader.GetDateTime(1);
writerCommand.CommandText = "UPDATE Dinners SET EventDate = '" + EventDate.AddDays(1).ToString() + "' WHERE DinnerID = " + DinnerID.ToString();
writerCommand.ExecuteNonQuery();
}
}
}
}
Your problem looks like something that you should be solving using T-SQL and not C#, unless there is some business rule that you are picking up dynamically and calculating the column values T-SQL should be the way to go. Just write a stored procedure or just open up Management studio and write the code to make your changes.
If this does not help then please elaborate on what exactly you want to do to the table, then we can help you figure out if this can be done via T-SQL or not.
[EDIT] you can do something like this
string sql = " USE " + paramDbName;
sql+= " ALTER TABLE XYZ ADD COLUMN " + param1 + " datatype etc, then put semicolon to separate the commands as well"
sql+= " UPDATE XYZ SET Columnx = " + some logic here
cmd.CommandText = sql;
cmd.ExecuteNonQuery();
get this executed on the required instance of Sql Server 2008.
If you have too many lines of text then use StringBuilder.
Here's a suggestion:
You can read data using a DataReader , create a update command for current row and add it to a list of commands.Then run update commands in a transaction.
something like this:
var commands=new List<SqlCommand>();
while(dr.Read())
{
var cmd=new SqlCommand();
cmd.CommandText="Add your command text here";
commands.Add(cmd);
}
using(var cnn=new SqlConnection("Connection String"))
{
IDbTransaction transaction;
try
{
cnn.Open();
transaction=cnn.BeginTransaction();
foreach(var cmd in commands)
{
cmd.Transaction=transaction;
cmd.ExecuteNonQuery();
cmd.Dispose();
}
transaction.Commit();
}
catch(SqlException)
{
if(transaction!=null)
transaction.Rollback();
throw;
}
}
What's the best way to check if a table exists in a Sql database in a database independant way?
I came up with:
bool exists;
const string sqlStatement = #"SELECT COUNT(*) FROM my_table";
try
{
using (OdbcCommand cmd = new OdbcCommand(sqlStatement, myOdbcConnection))
{
cmd.ExecuteScalar();
exists = true;
}
}
catch
{
exists = false;
}
Is there a better way to do this? This method will not work when the connection to the database fails. I've found ways for Sybase, SQL server, Oracle but nothing that works for all databases.
bool exists;
try
{
// ANSI SQL way. Works in PostgreSQL, MSSQL, MySQL.
var cmd = new OdbcCommand(
"select case when exists((select * from information_schema.tables where table_name = '" + tableName + "')) then 1 else 0 end");
exists = (int)cmd.ExecuteScalar() == 1;
}
catch
{
try
{
// Other RDBMS. Graceful degradation
exists = true;
var cmdOthers = new OdbcCommand("select 1 from " + tableName + " where 1 = 0");
cmdOthers.ExecuteNonQuery();
}
catch
{
exists = false;
}
}
If you're trying for database independence you will have to assume a minimum standard. IIRC The ANSI INFORMATION_SCHEMA views are required for ODBC conformance, so you could query against them like:
select count (*)
from information_schema.tables
where table_name = 'foobar'
Given that you are using ODBC, you can also use various ODBC API calls to retrieve this metadata as well.
Bear in mind that portability equates to write-once test anywhere so you are still going to have to test the application on every platform you intend to support. This means that you are inherently limited to a finite number of possible database platforms as you only have so much resource for testing.
The upshot is that you need to find a lowest common denominator for your application (which is quite a lot harder than it looks for SQL) or build a platform-dependent section where the non-portable functions can be plugged in on a per-platform basis.
I don't think that there exists one generic way that works for all Databases, since this is something very specific that depends on how the DB is built.
But, why do you want to do this using a specific query ?
Can't you abstract the implementation away from what you want to do ?
I mean: why not create a generic interface, which has among others, a method called 'TableExists( string tablename )' for instance.
Then, for each DBMS that you want to support , you create a class which implements this interface, and in the TableExists method, you write specific logic for this DBMS.
The SQLServer implementation will then contain a query which queries sysobjects.
In your application, you can have a factory class which creates the correct implementation for a given context, and then you just call the TableExists method.
For instance:
IMyInterface foo = MyFactory.CreateMyInterface (SupportedDbms.SqlServer);
if( foo.TableExists ("mytable") )
...
I think this is how I should do it.
I fully support Frederik Gheysels answer. If you have to support multiple database systems, you should implement your code against an abstract interface with specific implementations per database system. There are many more examples of incompatible syntax than just checking for an existing table (e.g.: limiting the query to a certain number of rows).
But if you really have to perform the check using the exception handling from your example, you should use the following query that is more efficient than a COUNT(*) because the database has no actual selection work to do:
SELECT 1 FROM my_table WHERE 1=2
I would avoid executing the select count(x) from xxxxxx as the DBMS will actually go ahead and do it which may take some time for a large table.
Instead just prepare a select * from mysterytable query. The prepare will fail if mysterytable does not exist. There is no need to actually execute the prepared statement.
The following works well for me...
private bool TableExists(SqlConnection conn, string database, string name)
{
string strCmd = null;
SqlCommand sqlCmd = null;
try
{
strCmd = "select case when exists((select '['+SCHEMA_NAME(schema_id)+'].['+name+']' As name FROM [" + database + "].sys.tables WHERE name = '" + name + "')) then 1 else 0 end";
sqlCmd = new SqlCommand(strCmd, conn);
return (int)sqlCmd.ExecuteScalar() == 1;
}
catch { return false; }
}
In current project on my job I need to write 'data agent' which would support a lot of database types.
So I decided to do next: write a base class with the base (database independent) functionality using virtual methods and override in subclasses all database-specific moments
Very Simple
use YOUR_DATABASE --OPTIONAL
SELECT count(*) as Exist from INFORMATION_SCHEMA.TABLES where table_name = 'YOUR_TABLE_NAME'
If the answer is 1, There is a table.
If the answer is 0, There is no table.
If you want to avoid try-catch solutions, I'm suggesting this method, using sys.tables
private bool IsTableExisting(string table)
{
string command = $"select * from sys.tables";
using (SqlConnection con = new SqlConnection(Constr))
using (SqlCommand com = new SqlCommand(command, con))
{
SqlDataReader reader = com.ExecuteReader();
while (reader.Read())
{
if (reader.GetString(0).ToLower() == table.ToLower())
return true;
}
reader.Close();
}
return false;
}