Is there a better way to dynamically build an SQL WHERE clause than by using 1=1 at its beginning?

I'm building some SQL query in C#. It will differ depending on some conditions stored as variables in the code.

string Query="SELECT * FROM Table1 WHERE 1=1 ";
if (condition1) 
    Query += "AND Col1=0 ";
if (condition2) 
    Query += "AND Col2=1 ";
if (condition3) 
    Query += "AND Col3=2 ";

It works, but testing 1=1 doesn't seem elegant. If I didn't use it, I would have to remember and check every time if "where" keyword was already added or not to the query.

Is there a nicer solution?

Save the conditions in a list:

List<string> conditions = new List<string>();

if (condition1) conditions.Add("Col1=0");
//...
if (conditions.Any())
    Query += " WHERE " + string.Join(" AND ", conditions.ToArray());

One solution is to simply not write queries manually by appending strings. You could use an ORM, like Entity Framework, and with LINQ to Entities use the features the language and framework offer you:

using (var dbContext = new MyDbContext())
{
	IQueryable<Table1Item> query = dbContext.Table1;
	
	if (condition1)
	{
		query = query.Where(c => c.Col1 == 0);
	}
	if (condition2)
	{
		query = query.Where(c => c.Col2 == 1);
	}
	if (condition3)
	{
		query = query.Where(c => c.Col3 == 2);
	}	
	
	PrintResults(query);
}

A slight bit of overkill in this simple case but I've used code similar to this in the past.

Create a function

string AddCondition(string clause, string appender, string condition)
{
    if (clause.Length <= 0)
    {
        return String.Format("WHERE {0}",condition);
    }
    return string.Format("{0} {1} {2}", clause, appender, condition);
}

Use it like this

string query = "SELECT * FROM Table1 {0}";
string whereClause = string.Empty;

if (condition 1)
    whereClause = AddCondition(whereClause, "AND", "Col=1");

if (condition 2)
    whereClause = AddCondition(whereClause, "AND", "Col2=2");

string finalQuery = String.Format(query, whereClause);

This way if no conditions are found you don't even bother loading a where statement in the query and save the sql server a micro-second of processing the junk where clause when it parses the sql statement.

There is another solution, which may also not be elegant, but works and solves the problem:

String query = "SELECT * FROM Table1";
List<string> conditions = new List<string>();
// ... fill the conditions
string joiner = " WHERE ";
foreach (string condition in conditions) {
  query += joiner + condition;
  joiner = " AND "
}

For:

• empty conditions list, the result will be simply SELECT * FROM Table1,
• a single condition it will be SELECT * FROM Table1 WHERE cond1
• each following condition will generate additional AND condN

Just do something like this:

using (var command = connection.CreateCommand())
{
    command.CommandText = "SELECT * FROM Table1";
    
    var conditions = "";
    if (condition1)
    {    
        conditions += "Col1=@val1 AND ";
        command.AddParameter("val1", 1);
    }
    if (condition2)
    {    
        conditions += "Col2=@val2 AND ";
        command.AddParameter("val2", 1);
    }
    if (condition3)
    {    
        conditions += "Col3=@val3 AND ";
        command.AddParameter("val3", 1);
    }
    if (conditions != "")
        command.CommandText += " WHERE " + conditions.Remove(conditions.Length - 5);
}

It's SQL injection safe and IMHO, it's pretty clean. The Remove() simply removes the last AND ;

It works both if no conditions have been set, if one have been set or if multiple have been set.

Just append two lines at back.

string Query="SELECT * FROM Table1 WHERE 1=1 ";
if (condition1) Query+="AND Col1=0 ";
if (condition2) Query+="AND Col2=1 ";
if (condition3) Query+="AND Col3=2 ";
Query.Replace("1=1 AND ", "");
Query.Replace(" WHERE 1=1 ", "");

E.g.

SELECT * FROM Table1 WHERE 1=1 AND Col1=0 AND Col2=1 AND Col3=2 

will become to

SELECT * FROM Table1 WHERE Col1=0 AND Col2=1 AND Col3=2 

While

SELECT * FROM Table1 WHERE 1=1 

will become to

SELECT * FROM Table1

=====================================

Thanks for pointing out a flaw of this solution:

"This could break the query if, for any reason, one of the conditions contains the text "1=1 AND " or " WHERE 1=1 ". This could be the case if the condition contains a subquery or tries to check if some column contains this text, for example. Maybe this isn't a problem in your case but you should keep it in mind… "

In order to get rid of this issue, we need to distinguish the "main" WHERE 1=1 and those from subquery, which is easy:

Simply make the "main" WHERE special: I would append a "$" sign

string Query="SELECT * FROM Table1 WHERE$ 1=1 ";
if (condition1) Query+="AND Col1=0 ";
if (condition2) Query+="AND Col2=1 ";
if (condition3) Query+="AND Col3=2 ";

Then still append two lines:

Query.Replace("WHERE$ 1=1 AND ", "WHERE ");
Query.Replace(" WHERE$ 1=1 ", "");

Use this:

string Query="SELECT * FROM Table1 WHERE ";
string QuerySub;
if (condition1) QuerySub+="AND Col1=0 ";
if (condition2) QuerySub+="AND Col2=1 ";
if (condition3) QuerySub+="AND Col3=2 ";

if (QuerySub.StartsWith("AND"))
    QuerySub = QuerySub.TrimStart("AND".ToCharArray());

Query = Query + QuerySub;

if (Query.EndsWith("WHERE "))
    Query = Query.TrimEnd("WHERE ".ToCharArray());

Why not using an existing Query Builder ? Something like Sql Kata.

It supports complex where conditions, joins and subqueries.

var query = new Query("Users").Where("Score", ">", 100).OrderByDesc("Score").Limit(100);
 
if(onlyActive)
{
   query.Where("Status", "active")
}

// or you can use the when statement

query.When(onlyActive, q => q.Where("Status", "active"))

it works with Sql Server, MySql and PostgreSql.

If this is SQL Server, you can make this code much cleaner.

This also assumes a known number of parameters, which may be a poor assumption when I think about the possibilities.

In C#, you would use:

using (SqlConnection conn = new SqlConnection("connection string"))
{
    conn.Open();
    SqlCommand command = new SqlCommand()
    {
        CommandText = "dbo.sample_proc",
        Connection = conn,
        CommandType = CommandType.StoredProcedure
    };

    if (condition1)
        command.Parameters.Add(new SqlParameter("Condition1", condition1Value));
    if (condition2)
        command.Parameters.Add(new SqlParameter("Condition2", condition2Value));
    if (condition3)
        command.Parameters.Add(new SqlParameter("Condition3", condition3Value));

    IDataReader reader = command.ExecuteReader();

    while(reader.Read())
    {
    }

    conn.Close();
}

And then on the SQL side:

CREATE PROCEDURE dbo.sample_proc
(
    --using varchar(50) generically
    -- "= NULL" makes them all optional parameters
    @Condition1 varchar(50) = NULL
    @Condition2 varchar(50) = NULL
    @Condition3 varchar(50) = NULL
)
AS
BEGIN
    /*
    check that the value of the parameter 
    matches the related column or that the 
    parameter value was not specified.  This
    works as long as you are not querying for 
    a specific column to be null.*/
    SELECT *
    FROM SampleTable
    WHERE (Col1 = @Condition1 OR @Condition1 IS NULL)
    AND   (Col2 = @Condition2 OR @Condition2 IS NULL)
    AND   (Col3 = @Condition3 OR @Condition3 IS NULL)
    OPTION (RECOMPILE)
    --OPTION(RECOMPILE) forces the query plan to remain effectively uncached
END

The quickest literal solution to what you're asking that I can think of is this:

string Query="SELECT * FROM Table1";
string Conditions = "";

if (condition1) Conditions+="AND Col1=0 ";
if (condition2) Conditions+="AND Col2=1 ";
if (condition3) Conditions+="AND Col3=2 ";

if (Conditions.Length > 0) 
  Query+=" WHERE " + Conditions.Substring(3);

It doesn't seem elegant, sure, to which I would refer you to CodeCaster's recommendation of using an ORM. But if you think about what this is doing here, you're really not worried about 'wasting' 4 characters of memory, and it's really quick for a computer to move a pointer 4 places.

If you have the time to learn how to use an ORM, it could really pay off for you. But in regards to this, if you're trying to keep that additional condition from hitting the SQL db, this will do it for you.

Depending on the condition, it might be possible to use boolean logic in the query. Something like this :

string Query="SELECT * FROM Table1  " +
             "WHERE (condition1 = @test1 AND Col1=0) "+
             "AND (condition2 = @test2 AND Col2=1) "+
             "AND (condition3 = @test3 AND Col3=2) ";

I like the fluent interface of stringbuilder, so I made some ExtensionMethods.

var query = new StringBuilder()
    .AppendLine("SELECT * FROM products")
    .AppendWhereIf(!String.IsNullOrEmpty(name), "name LIKE @name")
    .AppendWhereIf(category.HasValue, "category = @category")
    .AppendWhere("Deleted = @deleted")
    .ToString();

var p_name = GetParameter("@name", name);
var p_category = GetParameter("@category", category);
var p_deleted = GetParameter("@deleted", false);
var result = ExecuteDataTable(query, p_name, p_category, p_deleted);


// in a seperate static class for extensionmethods
public StringBuilder AppendLineIf(this StringBuilder sb, bool condition, string value)
{
    if(condition)
        sb.AppendLine(value);
    return sb;
}

public StringBuilder AppendWhereIf(this StringBuilder sb, bool condition, string value)
{
    if (condition)
        sb.AppendLineIf(condition, sb.HasWhere() ? " AND " : " WHERE " + value);
    return sb;
}

public StringBuilder AppendWhere(this StringBuilder sb, string value)
{
    sb.AppendWhereIf(true, value);
    return sb;
}

public bool HasWhere(this StringBuilder sb)
{
    var seperator = new string [] { Environment.NewLine };
    var lines = sb.ToString().Split(seperator, StringSplitOptions.None);
    return lines.Count > 0 && lines[lines.Count - 1].Contains("where", StringComparison.InvariantCultureIgnoreCase);
}

// http://stackoverflow.com/a/4217362/98491
public static bool Contains(this string source, string toCheck, StringComparison comp)
{
    return source.IndexOf(toCheck, comp) >= 0;
}

IMHO, I think that your approach is wrong:

Query the database by concatenating string is NEVER a good idea (risk of SQL injection and the code can easily be broken if you do some changes elsewhere).

You can use an ORM (I use NHibernate) or at least use SqlCommand.Parameters

If you absolutely want to use string concatenation, I would use a StringBuilder (it is the right object for string concatenation):

var query = new StringBuilder("SELECT * FROM Table1 WHERE");
int qLength = query.Length;//if you don't want to count :D
if (Condition1) query.Append(" Col1=0 AND");
if (Condition2) query.Append(" Col2=0 AND");
....
//if no condition remove WHERE or AND from query
query.Length -= query.Length == qLength ? 6 : 4;

As the last thought, Where 1=1 is really ugly but SQL Server will optimize it anyway.

The Dapper SqlBuilder is a pretty good option. It's even used in production on StackOverflow.

Read Sam's blog entry about it.

As far as I know, it's not part of any Nuget package, so you'll need to copy paste its code into your project or download the Dapper source and build the SqlBuilder project. Either way, you'll also need to reference Dapper for the DynamicParameters class.

I see this used all the time in Oracle while building dynamic SQL within stored procedures. I use it in queries while exploring data issues as well just to make switching between different filters of data faster... Just comment out a condition or add it back in easily.

I find it's pretty common and easy enough to understand to someone reviewing your code.

Using string function you can also do it this way:

string Query = "select * from Table1";

if (condition1) WhereClause += " Col1 = @param1 AND "; // <---- put conditional operator at the end
if (condition2) WhereClause += " Col1 = @param2 OR ";

WhereClause = WhereClause.Trim();

if (!string.IsNullOrEmpty(WhereClause))
    Query = Query + " WHERE " + WhereClause.Remove(WhereClause.LastIndexOf(" "));
// else
// no condition meets the criteria leave the QUERY without a WHERE clause  

I personally feel easy to to remove the conditional element(s) at the end, since its position is easy to predict.

I thought of a solution that, well, perhaps is somewhat more readable:

string query = String.Format("SELECT * FROM Table1 WHERE "
							 + "Col1 = {0} AND "
							 + "Col2 = {1} AND "
							 + "Col3 = {2}",
							(!condition1 ? "Col1" : "0"),
                            (!condition2 ? "Col2" : "1"),
                            (!condition3 ? "Col3" : "2"));

I'm just not sure whether the SQL interpreter will also optimize away the Col1 = Col1 condition (printed when condition1 is false).

public static class Ext
{
    public static string addCondition(this string str, bool condition, string statement)
    {
        if (!condition)
            return str;

        return str + (!str.Contains(" WHERE ") ? " WHERE " : " ") + statement;
    }

    public static string cleanCondition(this string str)
    {
        if (!str.Contains(" WHERE "))
            return str;

        return str.Replace(" WHERE AND ", " WHERE ").Replace(" WHERE OR ", " WHERE ");
    }
}

Realisation with extension methods.

    static void Main(string[] args)
    {
        string Query = "SELECT * FROM Table1";

        Query = Query.addCondition(true == false, "AND Column1 = 5")
            .addCondition(18 > 17, "AND Column2 = 7")
            .addCondition(42 == 1, "OR Column3 IN (5, 7, 9)")
            .addCondition(5 % 1 > 1 - 4, "AND Column4 = 67")
            .addCondition(Object.Equals(5, 5), "OR Column5 >= 0")
            .cleanCondition();

        Console.WriteLine(Query);
    }

Here is a more elegant way:

    private string BuildQuery()
    {
        string MethodResult = "";
        try
        {
            StringBuilder sb = new StringBuilder();

            sb.Append("SELECT * FROM Table1");

            List<string> Clauses = new List<string>();

            Clauses.Add("Col1 = 0");
            Clauses.Add("Col2 = 1");
            Clauses.Add("Col3 = 2");
            
            bool FirstPass = true;

            if(Clauses != null && Clauses.Count > 0)
            {
                foreach(string Clause in Clauses)
                {
                    if (FirstPass)
                    {
                        sb.Append(" WHERE ");

                        FirstPass = false;

                    }
                    else
                    {
                        sb.Append(" AND ");

                    }

                    sb.Append(Clause);

                }

            }

            MethodResult = sb.ToString();

        }
        catch //(Exception ex)
        {
            //ex.HandleException()
        }
        return MethodResult;
    }

As has been stated, creating SQL by concatenation is never a good idea. Not just because of SQL injection. Mostly because it's just ugly, difficult to maintain and totally unnecessary. You have to run your program with trace or debug to see what SQL it generates. If you use QueryFirst (disclaimer: which I wrote) the unhappy temptation is removed, and you can get straight in ta doin it in SQL.

This page has a comprehensive coverage of TSQL options for dynamically adding search predicates. The following option is handy for situations where you want to leave the choice of combinations of search predicates to your user.

select * from table1
where (col1 = @param1 or @param1 is null)
and (col2 = @param2 or @param2 is null)
and (col3 = @param3 or @param3 is null)
OPTION (RECOMPILE)

QueryFirst gives you C# null to db NULL, so you just call the Execute() method with nulls when appropriate, and it all just works. <opinion>Why are C# devs so reluctant to do stuff in SQL, even when it's simpler. Mind boggles.</opinion>

For longer filtering steps StringBuilder is the better approach as many says.

on your case I would go with:

StringBuilder sql = new StringBuilder();

if (condition1) 
    sql.Append("AND Col1=0 ");
if (condition2) 
    sql.Append("AND Col2=1 ");
if (condition3) 
    sql.Append("AND Col3=2 ");

string Query = "SELECT * FROM Table1 ";
if(sql.Length > 0)
 Query += string.Concat("WHERE ", sql.ToString().Substring(4)); //avoid first 4 chars, which is the 1st "AND "

Concise, elegant and sweet, as shown in the image below.