Share 6 useful MySQL statements

Today, we will introduce six useful MySQL SQL statements. Many may use PHP to implement these functions.

1. Calculate the number of years

You figured out how old this person is on your birthday.

SELECT DATE_FORMAT (FROM_DAYS (TO_DAYS (now ()-TO_DAYS (@ dateofbirth), % Y) + 0;

2. Difference between two time points

Returns the difference between two datetime values. Assume that dt1 and dt2 are of the datetime type, and their format is 'yyyy-mm-dd hh: mm: ss'. The difference between them is:

UNIX_TIMESTAMP (dt2)-UNIX_TIMESTAMP (dt1)

Dividing by 60 is the number of minutes in the difference, dividing by 3600 is the number of hours in the difference, and dividing by 24 is the number of days in the difference.

3. display the value of a column that appears N times

SELECT id

FROM tbl

Group by id

Having count (*) = N;

4. Calculate the workdays between two days

Workdays are divided into Saturday and Sunday holidays.

Select count (*)

FROM calendar

WHERE d BETWEEN Start AND Stop

And dayofweek (d) not in (1, 7)

AND holiday = 0;

5. Search for the primary key in the table

SELECT k. column_name

FROM information_schema.table_constraints t

JOIN information_schema.key_column_usage k

USING (constraint_name, table_schema, table_name)

WHERE t. constraint_type = PRIMARY KEY

AND t. table_schema = db

AND t. table_name = tbl

6. Check the size of your database.

SELECT

Table_schema AS Db Name,

Round (Sum (data_length + index_length)/1024/1024, 3) AS Db Size (MB ),

Round (Sum (data_free)/1024/1024, 3) AS Free Space (MB );

FROM information_schema.tables

Group by table_schema;

Hope to help you.