Summary of six useful SQL statements for MySQL database operations

In MySQL database operations, we often write some SQL statements to implement the functions we want. 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. If the number of years is calculated

In MySQL database operations, we often write some SQL statements to implement the functions we want. 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. If the number of years is calculated

In MySQL database operations, we often write some SQL statements to implement the functions we want. 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;

Here is an introduction to the useful SQL statements of the MySQL database.