MySQL查询，从一个表中查找并插入到第二个表中

Question

I am looking for the following:

• Need the row with this condition: Common values of Device columns AND first three chars of Interface column from both tables.

• Then the row which matched the above condition from Table1, retrieve the value of Specified column and store it in the Avgin column of the Table2 in the row where above condition matched.

Can someone help me with it? Database is MySQL.

Answer 1

If there are more than one matches, only the first one will be used. If there are none, null will be used. If you would like something else, use ifnull() .

UPDATE
  table2
SET
  avgin=ifnull(
    (
      SELECT
        Specified
      FROM
        table1
      WHERE
        table1.Device=table2.Device
        AND substring(table1.Interface,1,3)=substring(table2.Interface,1,3)
      LIMIT 1
    ),
    'default value'
  )

edit : added the ifnull()

Answer 2

UPDATE with JOIN is what you need here, something like this:

UPDATE Table2 AS t2
INNER JOIN table1 AS t1  ON LEFT(t2.Interface, 3) = LEFT(t1.Interface, 3)
                        AND t1.Device             = t2.Device
SET t2.Avgin = t1.specified;

With the JOIN condition, as you explained in your question:

 LEFT(t2.Interface, 3) = LEFT(t1.Interface, 3)
 AND 
 t1.Device             = t2.Device

LEFT will give you the first 3 chars from the left of both table.

See it in action here:

• SQL Fiddle Demo

This will make the table2 looks like:

|         CID |       DEVICE | INTERFACE |  AVGIN |
---------------------------------------------------
| HDC-HKG-R01 | HDC-TBONE-P1 |   P09/0/0 | 121.36 |
| OCB-OCD-R01 |      OCB-PE1 |     Gi5/2 |   0.17 |
| HDC-BSA-R01 | HDC-TBONE-P1 |   Se9/2/0 | (null) |

Answer 3

Use this to confirm you're getting the rows you're expecting (ie BEFORE updating anything):

SELECT
  t1.Specified

FROM
  table2 t2

  INNER JOIN table1 t1
  ON t1.device = t2.device
  AND LEFT(t1.interface,3) = LEFT(t2.interface,3)

And then, assuming that's right:

  UPDATE table2 t2

  INNER JOIN table1 t1
  ON t1.device = t2.device
  AND LEFT(t1.interface,3) = LEFT(t2.interface,3)

  SET t2.Avgin = ifnull(t1.specified,'Default Value For When t1.Specified is NULL')

Note we're using an INNER join... that means that rows from table2 that have no corresponding row in table1, are discarded from the results (which is what you want).

The IFNULL will allow you to use a default value in the case when your join succeeds (because device and first three chars of interface are common to both tables), but table1.specified has a NULL value for that row.