【MySQL】实例收集

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Leetcode – 627. Swap Salary

Given a table salary, such as the one below, that has m=male and f=female values. Swap all f and m values (i.e., change all f values to m and vice versa) with a single update statement and no intermediate temp table.

Note that you must write a single update statement, DO NOT write any select statement for this problem.

Example:

| id | name | sex | salary |
|----|------|-----|--------|
| 1  | A    | m   | 2500   |
| 2  | B    | f   | 1500   |
| 3  | C    | m   | 5500   |
| 4  | D    | f   | 500    |

After running your update statement, the above salary table should have the following rows:

| id | name | sex | salary |
|----|------|-----|--------|
| 1  | A    | f   | 2500   |
| 2  | B    | m   | 1500   |
| 3  | C    | f   | 5500   |
| 4  | D    | m   | 500    |

Solution:

Approach: Using UPDATE and CASE...WHEN [Accepted]

To dynamically set a value to a column, we can use UPDATE statement together when CASE...WHEN... flow control statement.

UPDATE salary
SET
    sex = CASE sex
        WHEN 'm' THEN 'f'
        ELSE 'm'
    END;

or

UPDATE salary SET sex = IF (sex = "m", "f", "m");

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找出最大（最新）日期（date/datetime）

Reference:
How to get rows with max date when grouping in MySQL?

SELECT t.id, m_t.MaxDTT, t.user_id
    FROM
    (SELECT MAX(login_datetime_record) AS MaxDTT, user_id
         FROM <table>
         GROUP BY user_id) AS m_t
    INNER JOIN <table> AS t
    ON t.user_id = m_t.user_id AND t.login_datetime_record = m_t.MaxDTT;

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Reference

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