3张表之间的SQL查询关系

Question

I have a scenario where I have the following tables:

Inventories delivery_items deliveries

I seek a query where, having the inventory id , I get the delivery_item (fk_inventory), which then I get the delivery from the (fk_delivery).

Manually, I go to the delivery_items table, then I search for the fk_inventory that matches the id from the inventory that I'm looking for, then I get the fk_delivery , and get the delivery . But I need to run a report on 15k+ items.

How to write a query where from a list of inventory ids I can get to the delivery following the relationship that I mentioned above?

Answer 1

There are many sites on writing SQL queries, differentiating between a normal (inner) join vs outer join, left join, right join, subqueries, etc. What you are looking to do is probably best (due to all inventory items in question) is simple joins.

Try to think of it this way, and maybe do it this way. Have a sheet of paper, one representing each table and write the columns on it.

Now, visually looking at the available tables, put them next to each other based on how they are related. Note the column in table A that is the foreign key to the next table. Then again, from the second to the third.

Once you have this done (or even if just mentally), you can SEE how they are related. This is the basis of the FROM clause

select *
   from
      YourFirstTable yft
         JOIN YourSecondTable yst
            on yft.WhateverKey = yst.MatchingKeyColumn
            JOIN YourThirdTable ytt
               on yst.KeyToThirdTable = ytt.KeyInThisTable

Now that you have all your relationships established, you can always declare the individual columns you want from those respective tables. Easier to use with the aliases such as I provided here via yft , yst , ytt representing the first, second and third tables. Use aliases appropriate to your tables such as i=inventories, di = delivery_items, d = deliveries.

Then add whatever FILTERING conditions you want. If the condition is based on the FIRST Table such as yft above, that would go into the WHERE clause such as

where yft.SomeColumn = 'blah'

If the filtering criteria is specific to your second or third table, just add that to the JOIN / ON condition so it stays with the table and you know contextually it is associated HERE. It makes it easier when you are getting into LEFT JOINs.

   from
      YourFirstTable yft
         JOIN YourSecondTable yst
            on yft.WhateverKey = yst.MatchingKeyColumn
            AND yst.SecondTableColumn = 'someOtherValue'
            AND yst.SomeOtherColumn = 'somethingElse'

So now, the engine can go through all inventory items, to the corresponding details, to the actual deliveries without having to do individual searches each time which would be painful to trace / run / and performance.