4.21 Database Aggregation functions

Aggregation Functions (1）Count（*）--only columns with numeric types can be summed（2）avg;--calculate the average score（3）Max;--Maximum（4）min;--Minimum（5） as;--alias from（6）Group  by--a group followed by a having--can only be used after group by（7） like;--What to do to delete a selectiontable Joins (1）Join..... on....;--find the composition of a two table arrangement（2） Right.....Join..... on;--The left table shows the whole, the right table has a relationship with the display, no relationship is empty. (3) Left.......Join...... on;

4.21 Database Aggregation functions