I'm using a left join in an Access query to get a table with a column added that only applies to a few rows. When I use a subquery as the table being joined it gives me a final table with the new column having the same value for all the rows. When I build a table that gives the same exact output as the subquery it works how I want it to. Why does a subquery and a table give me different results if they look the same.
I've looked over other questions about the difference between a table and a derived table(what I think I get using the subquery) and there doesn't seem to be a difference between the two.
SELECT *
FROM Table1
LEFT JOIN
(SELECT *, "P" AS PColumn FROM TableX, TableY WHERE TableX.x = TableY.y) AS Table2
ON (Table1.x = Table2.x)
WHERE Table1.X > 2
Every value in PColumn is "P". When I build a table that looks exactly like the subquery result and use that in place of the subquery, only row 5 has a PColumn value of "P" and the rest are null, which is what I want. The subquery and the table have the exact same values but different outputs in the outer SELECT statement
As inferred by the comments, this looks to be another occurrence of a bug present in the query optimiser of the JET database engine used by MS Access when evaluating outer joins, as described by Allen Browne in his article here.
Consider the following MCVE demonstrating the bug:
Table1
+----+
| ID |
+----+
| 1 |
| 2 |
+----+
Table2
+----+
| ID |
+----+
| 1 |
+----+
SQL
select * from table1 left join (select table2.id, "x" as X from table2) q on table1.id = q.id
Should return:
+-----------+------+---+
| table1.id | q.id | X |
+-----------+------+---+
| 1 | 1 | x |
| 2 | | |
+-----------+------+---+
Actually returns:
+-----------+------+---+
| table1.id | q.id | X |
+-----------+------+---+
| 1 | 1 | x |
| 2 | | x |
+-----------+------+---+
After confirming that my previous suggestions do not resolve the bug, instead leverage the bug to obtain expected results:
SELECT *
FROM Table1
LEFT JOIN
(SELECT TableX.x, Iif(TableX.x Is Null, Null, "P") AS PColumn
FROM TableX INNER JOIN TableY
ON TableX.x = TableY.y) AS Table2
ON (Table1.x = Table2.x)
WHERE Table1.X > 2
If Allen Brown is correct, the Access query optimizer executes a calculated field (i.e. static value) AFTER the join, so just make the field consider the existence of a normally non-null to reproduce correct outer join (i.e. left join) results.
I have the following:
TableA
ID | DocumentType | DocumentCode | DocumentDate | Warehouse | RefecenceCode
---+--------------+--------------+--------------+-----------+--------------
1 | DeliveryNote | DOC-001 | 2017-04-21 | 1 | NULL
2 | Invoice | DOC-002 | 2017-04-21 | 2 | DOC-001
As you can see, the warehouse is different on each document and DOC-002 is related to DOC-001 through the information in ReferenceCode column (which means that was created starting from DOC-001 as a source document).
It is supposed for the DOC-002 to have the same information but sometimes might be different and in this case, I was tried to create a query (I think self join applies here) in order to check what information is different in the DOC-002 in this case compared to DOC-001, based on the reference code, but I couldn't managed to do it.
If someone could give me a hand, I'll be very grateful.
This is the SQL query:
select *
from TableA tbl
inner join TableA tbla on tbl.id = tbla.id
where tbla.ReferenceCode = tbl.DocumentCode
You indeed want to join the table to itself. But joining on the ID column won't work, because that column doesn't relate records to each other. Instead, you need to join on the DocumentCode and ReferenceCode fields. Then only include the records that have some difference (in this case, I'm only comparing the DocumentDate and Warehouse fields).
select tbla.*
from TableA tbl
join TableA tbla on tbl.DocumentCode = tbla.ReferenceCode
where tbla.DocumentDate != tbl.DocumentDate
or tbla.Warehouse != tbl.Warehouse
What is the difference between a natural join and an inner join?
One significant difference between INNER JOIN and NATURAL JOIN is the number of columns returned.
Consider:
TableA TableB
+------------+----------+ +--------------------+
|Column1 | Column2 | |Column1 | Column3 |
+-----------------------+ +--------------------+
| 1 | 2 | | 1 | 3 |
+------------+----------+ +---------+----------+
The INNER JOIN of TableA and TableB on Column1 will return
SELECT * FROM TableA AS a INNER JOIN TableB AS b USING (Column1);
SELECT * FROM TableA AS a INNER JOIN TableB AS b ON a.Column1 = b.Column1;
+------------+-----------+---------------------+
| a.Column1 | a.Column2 | b.Column1| b.Column3|
+------------------------+---------------------+
| 1 | 2 | 1 | 3 |
+------------+-----------+----------+----------+
The NATURAL JOIN of TableA and TableB on Column1 will return:
SELECT * FROM TableA NATURAL JOIN TableB
+------------+----------+----------+
|Column1 | Column2 | Column3 |
+-----------------------+----------+
| 1 | 2 | 3 |
+------------+----------+----------+
The repeated column is avoided.
(AFAICT from the standard grammar, you can't specify the joining columns in a natural join; the join is strictly name-based. See also Wikipedia.)
(There's a cheat in the inner join output; the a. and b. parts would not be in the column names; you'd just have column1, column2, column1, column3 as the headings.)
An inner join is one where the matching row in the joined table is required for a row from the first table to be returned
An outer join is one where the matching row in the joined table is not required for a row from the first table to be returned
A natural join is a join (you can have either natural left or natural right) that assumes the join criteria to be where same-named columns in both table match
I would avoid using natural joins like the plague, because natural joins are:
not standard sql [SQL 92] and therefore not portable, not particularly readable (by most SQL coders) and possibly not supported by various tools/libraries
not informative; you can't tell what columns are being joined on without referring to the schema
your join conditions are invisibly vulnerable to schema changes - if there are multiple natural join columns and one such column is removed from a table, the query will still execute, but probably not correctly and this change in behaviour will be silent
hardly worth the effort; you're only saving about 10 seconds of typing
A natural join is just a shortcut to avoid typing, with a presumption that the join is simple and matches fields of the same name.
SELECT
*
FROM
table1
NATURAL JOIN
table2
-- implicitly uses `room_number` to join
Is the same as...
SELECT
*
FROM
table1
INNER JOIN
table2
ON table1.room_number = table2.room_number
What you can't do with the shortcut format, however, is more complex joins...
SELECT
*
FROM
table1
INNER JOIN
table2
ON (table1.room_number = table2.room_number)
OR (table1.room_number IS NULL AND table2.room_number IS NULL)
SQL is not faithful to the relational model in many ways. The result of a SQL query is not a relation because it may have columns with duplicate names, 'anonymous' (unnamed) columns, duplicate rows, nulls, etc. SQL doesn't treat tables as relations because it relies on column ordering etc.
The idea behind NATURAL JOIN in SQL is to make it easier to be more faithful to the relational model. The result of the NATURAL JOIN of two tables will have columns de-duplicated by name, hence no anonymous columns. Similarly, UNION CORRESPONDING and EXCEPT CORRESPONDING are provided to address SQL's dependence on column ordering in the legacy UNION syntax.
However, as with all programming techniques it requires discipline to be useful. One requirement for a successful NATURAL JOIN is consistently named columns, because joins are implied on columns with the same names (it is a shame that the syntax for renaming columns in SQL is verbose but the side effect is to encourage discipline when naming columns in base tables and VIEWs :)
Note a SQL NATURAL JOIN is an equi-join**, however this is no bar to usefulness. Consider that if NATURAL JOIN was the only join type supported in SQL it would still be relationally complete.
While it is indeed true that any NATURAL JOIN may be written using INNER JOIN and projection (SELECT), it is also true that any INNER JOIN may be written using product (CROSS JOIN) and restriction (WHERE); further note that a NATURAL JOIN between tables with no column names in common will give the same result as CROSS JOIN. So if you are only interested in results that are relations (and why ever not?!) then NATURAL JOIN is the only join type you need. Sure, it is true that from a language design perspective shorthands such as INNER JOIN and CROSS JOIN have their value, but also consider that almost any SQL query can be written in 10 syntactically different, but semantically equivalent, ways and this is what makes SQL optimizers so very hard to develop.
Here are some example queries (using the usual parts and suppliers database) that are semantically equivalent:
SELECT *
FROM S NATURAL JOIN SP;
-- Must disambiguate and 'project away' duplicate SNO attribute
SELECT S.SNO, SNAME, STATUS, CITY, PNO, QTY
FROM S INNER JOIN SP
USING (SNO);
-- Alternative projection
SELECT S.*, PNO, QTY
FROM S INNER JOIN SP
ON S.SNO = SP.SNO;
-- Same columns, different order == equivalent?!
SELECT SP.*, S.SNAME, S.STATUS, S.CITY
FROM S INNER JOIN SP
ON S.SNO = SP.SNO;
-- 'Old school'
SELECT S.*, PNO, QTY
FROM S, SP
WHERE S.SNO = SP.SNO;
** Relational natural join is not an equijoin, it is a projection of one. – philipxy
A NATURAL join is just short syntax for a specific INNER join -- or "equi-join" -- and, once the syntax is unwrapped, both represent the same Relational Algebra operation. It's not a "different kind" of join, as with the case of OUTER (LEFT/RIGHT) or CROSS joins.
See the equi-join section on Wikipedia:
A natural join offers a further specialization of equi-joins. The join predicate arises implicitly by comparing all columns in both tables that have the same column-names in the joined tables. The resulting joined table contains only one column for each pair of equally-named columns.
Most experts agree that NATURAL JOINs are dangerous and therefore strongly discourage their use. The danger comes from inadvertently adding a new column, named the same as another column ...
That is, all NATURAL joins may be written as INNER joins (but the converse is not true). To do so, just create the predicate explicitly -- e.g. USING or ON -- and, as Jonathan Leffler pointed out, select the desired result-set columns to avoid "duplicates" if desired.
Happy coding.
(The NATURAL keyword can also be applied to LEFT and RIGHT joins, and the same applies. A NATURAL LEFT/RIGHT join is just a short syntax for a specific LEFT/RIGHT join.)
Natural Join: It is combination or combined result of all the columns in the two tables.
It will return all rows of the first table with respect to the second table.
Inner Join: This join will work unless if any of the column name shall be sxame in two tables
A Natural Join is where 2 tables are joined on the basis of all common columns.
common column : is a column which has same name in both tables + has compatible datatypes in both the tables.
You can use only = operator
A Inner Join is where 2 tables are joined on the basis of common columns mentioned in the ON clause.
common column : is a column which has compatible datatypes in both the tables but need not have the same name.
You can use only any comparision operator like =, <=, >=, <, >, <>
Natural Join : A SQL Join clause combines fields from 2 or more tables in a relational database. A natural join is based on all columns in two tables that have the same name and selected rows from the two tables that have equal values in all matched columns.
--- The names and data types of both columns must be the same.
Using Clause : In a natural join,if the tables have columns with the same names but different data types, the join causes and error.To avoid this situation, the join clause can be modified with a USING clause. The USING clause specifies the columns that should be used for the join.
difference is that int the inner(equi/default)join and natural join that in the natuarl join common column win will be display in single time but inner/equi/default/simple join the common column will be display double time.
Inner join and natural join are almost same but there is a slight difference between them. The difference is in natural join no need to specify condition but in inner join condition is obligatory. If we do specify the condition in inner join , it resultant tables is like a cartesian product.
mysql> SELECT * FROM tb1 ;
+----+------+
| id | num |
+----+------+
| 6 | 60 |
| 7 | 70 |
| 8 | 80 |
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+----+------+
6 rows in set (0.00 sec)
mysql> SELECT * FROM tb2 ;
+----+------+
| id | num |
+----+------+
| 4 | 40 |
| 5 | 50 |
| 9 | 90 |
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+----+------+
6 rows in set (0.00 sec)
INNER JOIN :
mysql> SELECT * FROM tb1 JOIN tb2 ;
+----+------+----+------+
| id | num | id | num |
+----+------+----+------+
| 6 | 60 | 4 | 40 |
| 7 | 70 | 4 | 40 |
| 8 | 80 | 4 | 40 |
| 1 | 1 | 4 | 40 |
| 2 | 2 | 4 | 40 |
| 3 | 3 | 4 | 40 |
| 6 | 60 | 5 | 50 |
| 7 | 70 | 5 | 50 |
| 8 | 80 | 5 | 50 |
.......more......
return 36 rows in set (0.01 sec)
AND NATURAL JOIN :
mysql> SELECT * FROM tb1 NATURAL JOIN tb2 ;
+----+------+
| id | num |
+----+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+----+------+
3 rows in set (0.01 sec)
Inner join, join two table where column name is same.
Natural join, join two table where column name and data types are same.
I'm a SQL newbie, so please forgive the ignorance :)
Basically, I'm wondering what would be a good way of 'joining' 2 tables A and B wherein I just want to check if certain cases in A are in B. The thing is, Not all entries in A need to have matches in B, just a few. For example, Table A
merchant_id | tablet_id | address
33232 | 1 | 83 abs
94732 | 2 | 92 bcu
47373 | 3 | dkid
48238 | 3 | kdid
has joins with other tables in a query. In this same query, I want to implement a condition wherein if tablet_id in B matches with that of A, then to ignore those cases.
merchant | tablet_id | incentive?
33232 | 1 | Yes
67382 | 2 | No
Like I said, A and B only have a few cases in common. I tried a query with a JOIN between A & B and got nothing returned since a join might not be possible if there are no intersecting values between A & B. I'm just looking to implement an IF condition kind of thing.
Hopefully I was articulate. Any help would be appreciated!
SELECT * FROM `A` WHERE `tablet_id` NOT IN (SELECT `tablet_id` FROM `B`)
SELECT
*
FROM
A LEFT JOIN B
ON A.tablet_id = B.tablet_id
WHERE
B.tablet_id is null
You may be looking for the OUTER JOIN.
SELECT *
FROM TableA
LEFT OUTER JOIN TableB ON TableA.tablet_id = TableB.tabletID
This will return all rows from table A, and join rows from Table B where they meet the criteria in the ON clause. If no row exists in Table B for a row in Table A, the Table B column values in the results will be NULL.
I have 2 tables that I am trying to combine in a specific way
Table 1: ‘part_defs’ Table 2 Items_part_values
in value_defs:
ID | Name
-------------
1 | color
2 | size
3 | weight
in Items_part_values
ItemID | valueID | Value
-------------------------
10 | 1 | red
11 | 1 | blue
What I need is a query where for a given item all the rows from value_defs appear and if they have a value in Items_part_values the value.
So for Item 11 I want
ID | Name | Value
--------------------
1 | color | red
2 | size | NULL
3 | weight | NULL
I’m new to MySQL, in access I would have created a subquery with the ItemID as a parameter and then done a Left Join with value_defs on the result.
Is there a way of doing something similar in MySQL?
Thanks
Use:
SELECT p.id,
p.name,
ipv.value
FROM PART_DEFS p
LEFT JOIN ITEMS_PART_VALUES ipv ON ipv.valueid = p.id
AND ipv.itemid = ?
Replace the "?" with the itemid you want to search for.
This means all the PARTS_DEF rows will be returned, and if the ITEMS_PART_VALUES.valueid matches the PART_DEFS.id value, then the ITEMS_PART_VALUES.value value will be displayed for the item you are looking for. If there's no match, the value column will be NULL for that record.
There's a difference in OUTER JOINs, when specifying criteria in the JOIN vs the WHERE clause. In the JOIN, the criteria is applied before the JOIN occurs while in the WHERE clause the criteria is applied after the JOIN.
Use a left join:
SELECT * FROM Table1 LEFT JOIN Table2 USING (ID);
Edit:
SELECT * FROM part_defs LEFT JOIN Items_part_values ON part_defs.ID = Items_part_values.valueID;