SQL 101: A Data Definition Language Hands-on Tour, Part 5

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Today I’ll be talking about views. Not grandiose landscapes, but SQL views. If you think they’re just another name for logical files, you’re wrong. Keep reading to find out more!

Last time around, I talked about tables and how they are, at the same time, similar to and (very) different from physical files. Now I’ll discuss how views are also similar to and (very) different from logical files. If you haven’t read the previous articles (here and here), please do so, because the tables described there are the basis for the code in this article. As promised in the last TechTip, here’s the structure for the warehouse shelf master table:

CREATE TABLE MYSCHEMA.TBL_WAREHOUSE_SHELF_MASTER

FOR SYSTEM NAME WSMST

(WAREHOUSE_ID           FOR COLUMN WHID

DEC ( 8, 0)            NOT NULL WITH DEFAULT,

SHELF_ID               FOR COLUMN SHELFID

DEC ( 12, 0)           NOT NULL WITH DEFAULT,

SHELF_HEIGHT           FOR COLUMN SHLHEIGHT

DEC ( 4, 2)            NOT NULL WITH DEFAULT,

SHELF_WIDTH            FOR COLUMN SHLWIDTH

DEC ( 4, 2)            NOT NULL WITH DEFAULT,

SHELF_DEPTH            FOR COLUMN SHLDEPTH

DEC ( 4, 2)            NOT NULL WITH DEFAULT,

SHELF_REMARKS    FOR COLUMN SHL_RMK

VARCHAR ( 120)   NOT NULL WITH DEFAULT,

PRIMARY KEY (WAREHOUSE_ID, SHELF_ID))

RCDFMT WSMSTR

With that, you should have all the tables described in the previous TechTips (tables TBL_WAREHOUSE_MASTER, TBL_WAREHOUSE_SHELF_MASTER, TBL_ITEM_MASTER, and TBL_INVENTORY_MASTER). Now I can insert a couple of items, warehouses, and warehouse shelf definitions in the respective master tables so that I can finally add an item to my inventory. If I want to list that inventory item, however, InvMst is not enough. The item and warehouse descriptions are not shown because they’re in separate tables. You already know how to solve this, right? Just write a SELECT statement with an inner join, and you’re done—but you’ll need to add that inner join to all the SELECT statements that need the item and warehouse descriptions, which is not very friendly. That’s where the view concept comes into play. You already know that a view is similar to a non-keyed logical file; now let’s see how to create one. SQL’s VIEW syntax is the following:

CREATE VIEW <view name> FOR SYSTEM NAME <view system name>

(<list of the view’s column names separated by commas>)

AS

<valid Select statement, containing the same number of columns as specified in the view’s column names list>

RCDFMT <record format name>

I know it looks a bit messy, but it’s actually simple. Here’s an example of a view containing the data from the inventory master table, plus the item and warehouse descriptions:

CREATE VIEW VIEW_ITEMS_IN_INVENTORY FOR SYSTEM NAME V_INVMST01

(ITEM_ID, ITEM_DESCRIPTION, LOT_NUMBER, EXPIRATION_DATE, WAREHOUSE_ID, WAREHOUSE_NAME, SHELF_ID, ITEM_IN_UNITS, ITEM_QUANTITY)

AS

SELECT      INV.ITEM_ID

            , ITM.ITEM_DESC

            , INV.LOTNBR

            , INV.EXPDATE

            , INV.WHID

            , WH.WAREHOUSE_NAME

            , INV.SHELF_ID

            , INV.ITEMUN

            , INV.ITEMQTY

FROM        MYSCHEMA.INVMST

INNER JOIN  ITMMST ITM ON INV.ITEM_ID = ITM.ITEM_ID

INNER JOIN  WHMST WH ON INV.WHID = WH.WHID

RCDFMT INVMST01R

This statement has three parts:

  • The VIEW identification
  • The list of column names, which can be different from the “original” column names that come from the third part of the VIEW statement
  • A valid SELECT statement, as simple or complex as you’d like, that supplies a number of columns equal to that in the view’s column names

The SELECT statement looks a bit complex because of the inner joins, but it doesn’t include a WHERE clause, so every item in inventory is accessible using this view. You’d use it as you’d use a logical file, in the SQL or RPG environments. Note that I didn’t specify “system names” for the view’s columns, so in an RPG context, the column names would appear a bit garbled, with the first four characters and a 00001 suffix, instead of the full name of the column. I can create a simpler view containing all the columns of a certain table, but not all the records. For instance, if I want a view over the InvMst table that includes inventory items from warehouse 333 and don’t want to rename the “original” columns, I can simply write this:

CREATE VIEW VIEW_WAREHOUSE_333_INVENTORY FOR SYSTEM NAME V_INVMST02

AS

SELECT      *

FROM        MYSCHEMA.INVMST

WHERE       WHID = 333

RCDFMT INVMST02R

As you can see, this is a much shorter view definition. It contains the bare minimums (the view SQL name and a SELECT statement), plus the view’s system name and an alternate record format name. From these two examples, you’ve seen that views are similar to logical files. In fact, this last one is similar to a logical file with a (S)elect option over InvMst. However, unlike a logical file, an SQL VIEW doesn’t provide a built-in index; you need to create your own. That’s what we’ll be talking about in the next TechTip.

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