MySQL common commands

One 、 summary

Summarize some common commands

Two 、 command

2.1 Copy old table to new table

CREATE TABLE new_table_name
SELECT * FROM old_table_name;

2.2 Delete data in table

delete from table_name;

2.3 Add a number to the result

SET @row_number = 0;
SELECT (@row_number := @row_number + 1 ) AS num
FROM table_name

2.4 The minimum number of query groups id

SELECT min(book_id), book_name, book_type 
FROM t_book 
GROUP BY book_name, book_type #  Group according to two fields 

2.5 Delete duplicate records , The one that retains the smallest index

t_test The data in the table ：

id	book_name	book_type
1	The age of losers	noval
2	Warring States astray	noval
3	The age of losers	noval
4	Warring States astray	noval
5	The age of losers	essay

Duplicate records are uniquely identified by two fields .

therefore id by 1 The record of and id by 3 The record of is a duplicate record , And id by 5 The record of is not a duplicate record , Because of their book_type Different , One is noval, One is essay.

So what we want is to delete 3、4 Two lines of records , They repeated .

DELETE 
FROM   t_test
WHERE  (book_name, book_type) IN (
       SELECT *
       FROM   (SELECT   book_name, book_type
               FROM     t_test
               GROUP BY book_name, book_type
               HAVING   count(*) > 1) A)
AND   id  NOT IN (
        SELECT *
        FROM   (SELECT   min(id)
                FROM     t_test
                GROUP BY book_name, book_type
                HAVING   count(*) > 1) B)
ORDER BY book_name, book_type;

1. Get the smallest after grouping first id,
2. Delete duplicate records and this id It cannot be the smallest id, So keep the smallest id The record of .

result ：

id	book_name	book_type
1	The age of losers	noval
2	Warring States astray	noval
5	The age of losers	essay