如何从数据库中选择倒数第二行和最后一行？

Question

When we are retrieving data from database, we use something like this

SELECT * FROM Names

However, we will get all the data inside the specific table. Since I am going to update some data to the database and want to make a comparison bewteen the last row of data in the db and the most updated data, how can I select and retrieve the last two row of the database only?

Answer 1

SQL Server syntax:

SELECT TOP 2 column_name FROM table_name ORDER BY column_name DESC;  

Example:

If you want to retrieve the last value of the customer_name column from the customers table, you need to write the following query:

SELECT LAST (CUSTOMER_NAME) AS LAST_CUSTOMER FROM CUSTOMERS;  

Answer 2

如果您使用的是 SQL Server，您将执行以下操作：

SELECT TOP 2 * FROM Names ORDER BY Name DESC

Answer 3

You should include a column in the Names table to keep track of when a name was added to the table since you cannot guarantee that the rows are in the order that they were inserted. With that column you can use the order by clause..

In MySQL Syntax:

SELECT *
FROM Names
ORDER BY order_column DESC
LIMIT 2;

If you want to get the last rows as they are, you cannot order the table by the inserted names because that is just getting the 2 names that come last in an alphabetically sorted list of names. You could try something like this where you include an offset in the limit clause if you get the number of rows in the table:

SELECT *
FROM Names
LIMIT *num_rows*-2, 2;

You would have to know the number of rows to use this query or use an additional query to implement a row count that works with limit. This, however, still may not be accurate since the system may not order the rows as they were inserted. I still recommend the first option where you keep track of order/time a name was inserted.