Six useful SQL statements for MySQL database operations-MySQL

Summary of six useful SQL statements for MySQL database operations bitsCN.com in MySQL database operations, we often write some SQL statements to implement the desired functions. However, it seems difficult for beginners of MySQL databases. This article summarizes six useful SQL statements. beginners can use the following format. let's take a look at this part.
1. calculate the number of years
If you want to calculate the person's age by birthday, you can use the following statement:

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 are in the format of 'yyyy-mm-dd hh: mm: SS'. The difference between them is in the number of seconds: UNIX_TIMESTAMP (dt2) -UNIX_TIMESTAMP (dt1) divided by 60 is the number of minutes in the difference, divided by 3600 is the number of hours in the difference, and then divided 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;

This is the introduction of useful SQL statements for MySQL databases. if you want to learn more about MySQL databases, you can refer to the article http://www.bitscn.com/list/list_assist_1.htm, which will surely bring you the opportunity! BitsCN.com