Operation when a select query is in MySQL

Select is the function of querying the data in a table in MySQL, but it can also be done directly at query time, and then return the result after the query.

Like what

1 SELECT user, Math*(Ifnull (Adven,0 from

The Ifnull function is to judge the Adven data, and if the data in Adven is not NULL, the data of the Adven itself is returned, and if NULL, 0 is returned.

Other +,-,/operations can also be replaced with the code above.

More functions

function	Description
AVG (column)	Returns the average of a column
COUNT (column)	Returns the number of rows in a column (not including NULL values)
COUNT (*)	Returns the number of rows selected
First (column)	Returns the value of the first record in a specified field
Last (column)	Returns the value of the last record in the specified field
MAX (column)	Returns the highest value of a column
MIN (column)	Returns the lowest value of a column
STDEV (column)
STDEVP (column)
SUM (column)	Returns the sum of a column
VAR (column)
VARP (column)

GROUP by statement

The GROUP BY statement is used to combine aggregate functions to group result sets based on one or more columns.

We have the following "Orders" table:

o_id	OrderDate	Orderprice	Customer
1	2008/12/29	1000	Bush
2	2008/11/23	1600	Carter
3	2008/10/05	700	Bush
4	2008/09/28	300	Bush
5	2008/08/06	2000	Adams
6	2008/07/21	100	Carter

Now we want to find the total amount (total order) for each customer.

We want to use GROUP by statements to group customers.

We use the following SQL statements:

1 SELECT Customer,SUM from Orders2GROUP by Customer

The result set looks like this:

Customer	SUM (Orderprice)
Bush	2000
Carter	1700
Adams	2000

Operation when a select query is in MySQL