MySQL选择只有值的列

Question

I found a similar question but not quite the answer that I need. That's why I'm asking again. Thank you for understanding.

I have a table that has 100+ columns.

After researching I found this:

Select * from your_table 
WHERE col1 and col2 and col3 and col4 and col5 IS NOT NULL

My question is, is there any way that I can filter out all the columns that has no data wihout listing them all like the code above?

Answer 1

A way of doing this is to use the fact that a concat of columns where any of the columns is null results in a null result. MD5 hashing the columns can then be tested - you still have to specify all the appropriate columns but that is trivial.

DROP TABLE IF EXISTS T;

CREATE TABLE T
(ID INT, COL1 INT,COL2 VARCHAR(2));
INSERT INTO T VALUES
(1,NULL,NULL),
(2,1,NULL),
(3,NULL,'AA'),
(4,1,'BB');


SELECT ID,COL1,COL2,concat(col1,col2) ccol,MD5(CONCAT(COL1,COL2)) hsh
FROM T
;

+------+------+------+------+----------------------------------+
| ID   | COL1 | COL2 | ccol | hsh                              |
+------+------+------+------+----------------------------------+
|    1 | NULL | NULL | NULL | NULL                             |
|    2 |    1 | NULL | NULL | NULL                             |
|    3 | NULL | AA   | NULL | NULL                             |
|    4 |    1 | BB   | 1BB  | b2161dc77696ff2fd06d9f814b443923 |
+------+------+------+------+----------------------------------+
4 rows in set (0.001 sec)

SELECT ID,COL1,COL2,MD5(CONCAT(COL1,COL2))
FROM T
WHERE MD5(CONCAT(COL1,COL2)) IS NOT NULL;

+------+------+------+----------------------------------+
| ID   | COL1 | COL2 | MD5(CONCAT(COL1,COL2))           |
+------+------+------+----------------------------------+
|    4 |    1 | BB   | b2161dc77696ff2fd06d9f814b443923 |
+------+------+------+----------------------------------+
1 row in set (0.009 sec)

If you know the columns you wish to exclude you can build a sql statement from information_schema.columns

select concat('select *', ' from t where md5(concat(', (select group_Concat(column_name) from information_schema.columns where table_schema = 'sandbox' and table_name = 't' and column_name <> 'id'), ')) is not null;' );

Copy,paste and execute this or

set @sql = (
select concat('select *',
' from t where md5(concat(',
(select group_Concat(column_name) 
                    from information_schema.columns 
                    where table_schema = 'sandbox' and table_name = 't' and column_name <> 'id'),
                    ')) is not null;'
            ));
        
prepare sqlstmt from @sql;
execute sqlstmt;
deallocate prepare sqlstmt;

Answer 2

This code uses the following provider.

Imports MySql.Data.MySqlClient

First I got the column names for the fields in the table. I only selected fields that are nullable to begin with.

Next I built the Select string by looping through the column names and adding the phrases to a StringBuilder . (more efficient when building a string of over 10 pieces)

Finally retrieve the records that meet your requirements.

Private Function GetNullableColumnNames() As DataTable
    Dim MyTableSchema As New DataTable
    Using cn As New MySqlConnection(My.Settings.StudentConnection),
            cmd As New MySqlCommand("SELECT COLUMN_NAME
                                    FROM INFORMATION_SCHEMA.COLUMNS
                                    WHERE table_name = 'students'
                                    AND table_schema = 'student'
                                    AND IS_NULLABLE = 'YES';", cn)
        cn.Open()
        MyTableSchema.Load(cmd.ExecuteReader)
    End Using
    Return MyTableSchema
End Function

Private Function GetSelectString(dt As DataTable) As String
    Dim sb As New StringBuilder
    sb.Append("Select * From students Where 1 = 1")
    For Each row As DataRow In dt.Rows
        sb.Append($" And {row(0)} IS NOT NULL")
    Next
    sb.Append(";"
    Debug.Print(sb.ToString)
    Return sb.ToString
End Function

Private Function GetRecords(sql As String) As DataTable
    Dim dt As New DataTable
    Using cn As New MySqlConnection(My.Settings.StudentConnection),
            cmd As New MySqlCommand(sql, cn)
        cn.Open()
        dt.Load(cmd.ExecuteReader)
    End Using
    Return dt
End Function

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    Dim dt = GetNullableColumnNames()
    Dim sql = GetSelectString(dt)
    Dim dt2 = GetRecords(sql)
    DataGridView1.DataSource = dt2
End Sub

Answer 3

you can do this:

Select * from table_name 
WHERE NULL NOT IN (column1, column2, ...);