检查记录是否存在，如果字段为null，则始终返回false

Question

So I have the following situation:

as you can see some fields are null, so I want check before insert the record is this already exists inside the table goal , the record that I'm going to insert contains exactly the same structure of the record already avaialble in the table.

This is my code:

public bool CheckGoalExist(Goal goal, Goal.GoalType type, int matchId)
{
    using (MySqlConnection connection = new DBConnection().Connect())
    {
        using (MySqlCommand command = new MySqlCommand())
        {
            command.Connection = connection;
            command.CommandText = "SELECT COUNT(*) FROM goal " +
                "WHERE player_marker_id = @player_marker_id AND " +
                "team_id = @team_id AND " +
                "player_assist_id = @player_assist_id AND " +
                "match_id = @match_id AND " +
                    "minute = @minute AND " +
                    "type = @type";

            command.Parameters.AddWithValue("@team_id", goal.TeamId);
            command.Parameters.AddWithValue("@player_marker_id", goal.MarkerPlayer.Id);
            command.Parameters.AddWithValue("@player_assist_id", goal.AssistPlayer?.Id);
            command.Parameters.AddWithValue("@match_id", matchId);
            command.Parameters.AddWithValue("@minute", goal.Minute);
            command.Parameters.AddWithValue("@type", GetGoalTypeId(type));

            return Convert.ToBoolean(command.ExecuteScalar());
        }
    }
}

this will return false but the value of goal are this:

TeamId = 95
MarkerPlayer.Id = 122
AssistPlaer = null
matchId = 2564940
Minute = 82'
Type = 5

why return false?

Answer 1

If AssistPlaer is null , then you cannot use = . You need to check if the parameter is null first. Here's a common approach with an or statement:

command.CommandText = "SELECT COUNT(*) FROM goal " +
            "WHERE player_marker_id = @player_marker_id AND " +
            "team_id = @team_id AND " +
            "(@player_assist_id is null or player_assist_id = @player_assist_id) AND " +
            "match_id = @match_id AND " +
                "minute = @minute AND " +
                "type = @type";

You may need to do that for other potential null values as well.

Answer 2

since "AssistPlaer" is "NULL", the query in SQL cannot use the equal operator "=", but must compare to "NULL" using the "IS" or "IS NOT".

your query states:

player_assist_id = @player_assist_id

but "NULL" values do not respond to equal operators, the only way to test if it's null or not is this:

player_assist_id IS NULL

so in your query you could bypass that using something like:

(@player_assist_id IS NULL AND player_assist_id IS NULL) OR (player_assist_id = @player_assist_id)

apply this behavior to any column that can contain "NULL".

Answer 3

如果您不知道该属性值是否为NULL，则可以使用IFNULL字符串函数，以便它将NULL值替换为0或您在该特定列中定义的某个其他值。