How can I find which record in a table has most fields populated?

Question

Or perhaps put another way: How can I find which record in a table has the least null or blank fields?

Without having to count every single field individually that is - the table has 161 fields.

Answer 1

This can be done by constructing a dynamic query like:

SELECT id,
       (IF(col1 = NULL OR col1 = "", 1, 0) +
        IF(col2 = NULL OR col3 = "", 1, 0) +
        ...
        IF(coln = NULL OR coln = "", 1, 0)
       ) AS null_count
FROM table_name
ORDER BY null_count DESC
LIMIT 1;

This can be easily done by forming a new dynamic query using INFORMATION_SCHEMA.COLUMNS and then executing dynamic sql query . Also you might need to increase the maximum length of output from GROUP_CONCAT function by setting session level variable group_concat_max_len to higher value.

SET GLOBAL group_concat_max_len = 4294967295;

SELECT @query1 := CONCAT('SELECT id,
                                 (',
                         GROUP_CONCAT(CONCAT('IF(',COLUMN_NAME,' IS NULL OR ',
                                                   COLUMN_NAME,' = "", 1, 0
                                                ) 
                                           ') 
                                      SEPARATOR ' + '),
                                ') AS null_count
                         FROM table_name
                         ORDER BY null_count DESC
                         LIMIT 1')
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_SCHEMA = SCHEMA()
      AND TABLE_NAME = 'table_name';

PREPARE stmt FROM @query1; EXECUTE stmt; DEALLOCATE PREPARE stmt;

Example: SQLFiddle

Answer 2

DISCLAIMER: I got the question wrong, thought the OP was looking for the column of a table with the most NULLs. Nonetheless, maybe it's useful for anyone.

Create a procedure like this:

drop procedure if exists test_most_pop_field;
DELIMITER $$
CREATE PROCEDURE test_most_pop_field(IN tableName varchar(100))
BEGIN

DECLARE done INT DEFAULT 0;
DECLARE sql_query VARCHAR(255);

DECLARE cur CURSOR FOR 
SELECT CONCAT('INSERT INTO tmp_result(columnName, numberOfEmptyRows) SELECT "', COLUMN_NAME, '" AS columnName, SUM(IF(',COLUMN_NAME,' IS NULL OR ', COLUMN_NAME,' = "", 1, 0)) AS numberEmptyRows FROM ', TABLE_NAME)
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_SCHEMA = SCHEMA()
AND TABLE_NAME = tableName;

DECLARE CONTINUE HANDLER FOR SQLSTATE '02000' SET done = 1;

DROP TABLE IF EXISTS tmp_result;
CREATE TEMPORARY TABLE tmp_result(columnName varchar(100), numberOfEmptyRows int);

OPEN cur;

REPEAT
  FETCH cur INTO sql_query;
  IF NOT done THEN
    BEGIN
      SET @sql = sql_query; /*this extra step is necessary, cause otherwise it's a syntax error, don't ask me why*/
      PREPARE stmt FROM @sql; 
      EXECUTE stmt; 
      DEALLOCATE PREPARE stmt;
    END;
  END IF;
UNTIL done END REPEAT;

CLOSE cur;

SELECT * FROM tmp_result ORDER BY numberOfEmptyRows DESC /*optionally LIMIT 1*/;

END $$
DELIMITER ;

Then call it with the table name you want to examine:

CALL test_most_pop_field('yourTableName');