如何编写一个MySQL查询，该查询返回一个临时列，该列包含有关与该行相关的项目是否在另一个表中的标志

Question

How might I write a MySQL query to achieve the desired result shown below?

I have two tables:

TABLE_USERS:

 ID | Name  |  ... 
--------------------
 1  | Ash   |
 2  | Tim   |
 3  | Jim   |
 4  | Jay   |
 5  | Tom   |


TABLE_FLAGS:

 ID | Reason |  ...
----------------------
 2  |  ??    |
 4  |  ...   |

I want to know how to write a query that yields a result with the following columns:

DESIRED RESULT:


 ID | Name  | Flagged
----------------------
 1  | Ash   |  false
 2  | Tim   |  true
 3  | Jim   |  false
 4  | Jay   |  true
 5  | Tom   |  false

I could do:

SELECT TABLE_USERS.ID, TABLE_USERS.NAME
FROM TABLE_USER

To get the first two columns, but I'm not sure how to get the last one...

The final column does not directly correspond to a column in one of the two tables; instead for each row it returns true or false base on whether an entry for that rows id value exists in TABLE_FLAGS

Thanks

Answer 1

   SELECT   tu.ID, 
            tu.NAME,
            CASE WHEN tf.ID IS NOT NULL THEN 'true' ELSE 'false' END AS Flagged
      FROM  TABLE_USER tu
            LEFT OUTER JOIN TABLE_FLAGS tf ON tu.ID = tf.ID

Answer 2

This will show your desired output:

SELECT tu.ID, tu.NAME,
       CASE WHEN tf.ID IS NOT NULL THEN 'true' ELSE 'false' END AS Flagged
  FROM TABLE_USER tu
       LEFT OUTER JOIN TABLE_FLAGS tf ON tu.ID = tf.ID ;