mysql field name from variable

Just as a heads up to these correct answers you can also do it inside a stored procedure this worked perfectly for me in MySQL 8x Community:

CREATE DEFINER=`root`@`127.0.0.1` PROCEDURE `SP_LIST_COLLECTORS`(
    IN P_email VARCHAR(60),#Admin email
    IN P_password_hash VARCHAR(255),#Admin hash
    IN P_filter_field VARCHAR(80),
    IN P_filter_value VARCHAR(255)
)
BEGIN
DECLARE V_filter_field VARCHAR(80);
    SET V_filter_field = P_filter_field;
    BEGIN
        GET DIAGNOSTICS CONDITION 1 @ERRNO = MYSQL_ERRNO, @MESSAGE_TEXT = MESSAGE_TEXT;
        SELECT 'ERROR' AS STATUS, CONCAT('MySQL ERROR: ', @ERRNO, ': ', @MESSAGE_TEXT) AS MESSAGE;
    END;
    SET @statement = CONCAT('SELECT collector_id, email, address, post_code, phone, alt_phone, contact_name
    FROM collectors_table
    WHERE ',P_filter_field, '=\'', P_filter_value, '\';');
    #SELECT collector_id, email, address, post_code, phone, alt_phone, contact_name FROM collectors_table WHERE (V_filter_field) = P_filter_value;
    PREPARE stmnt FROM @statement;
    EXECUTE stmnt;
END

If the string is in your external application (like PHP), sure, just construct the MySQL statement.

If the string is inside a MySQL table, you can't. MySQL has no eval() or such function. The following is impossible:

Suppose you have a table queries with a field columnname that refers to one of the column names in the table mytable. There might be additional columns in queries that allow you to select the columnname you want.

INSERT INTO queries  (columname) VALUES ("name")
SELECT (select columnname from queries) from mytable

You can however work with PREPARED STATEMENTS. Be aware this is very hacky.

SELECT columnname from queries into @colname;
SET @table = 'mytable';
SET @s = CONCAT('SELECT ',@colname,' FROM ', @table);

PREPARE stmt FROM @s;
EXECUTE stmt;

If you want to select more than one column:

SELECT GROUP_CONCAT(COLUMN_NAME) FROM information_schema.`COLUMNS` C 
WHERE table_name = 'MyTb' AND COLUMN_NAME LIKE '%whatever%' INTO @COLUMNS;

SET @table = 'MyTb';
SET @s = CONCAT('SELECT ',@columns,' FROM ', @table);

PREPARE stmt FROM @s;
EXECUTE stmt;