My SQL select max, min, average of column ...

https://stackoverflow.com/questions/14787561/find-the-average-of-two-combined-columns-in-sql
https://stackoverflow.com/questions/8752705/calculate-average-of-column-from-mysql-query

https://stackoverflow.com/questions/18748830/select-max-min-last-value-from-a-column-in-mysql-table

https://stackoverflow.com/questions/3590530/mysql-select-multiple-maximum-values

https://stackoverflow.com/questions/2612914/how-to-make-mysql-query-results-order-by-conditions-order

https://stackoverflow.com/questions/232935/sql-query-where-field-does-not-contain-x

number-of-unique-values
https://stackoverflow.com/questions/16697215/mysql-count-number-of-unique-values
SELECT count( DISTINCT(email) ) FROM orders

http://www.techfounder.net/2009/02/02/selecting-closest-values-in-mysql/