Below is a join based on where clause:
SELECT a.* FROM TEST_TABLE1 a,TEST_TABLE2 b,TEST_TABLE3 c
WHERE a.COL11 = b.COL11
AND b.COL12 = c.COL12
AND a.COL3 = c.COL13;
I have been learning SQL from online resources and trying to convert it with joins
Two issues:
The original query is confusing. The outer joins (with the (+) suffix) are made irrelevant by the last where condition. Because of that condition, the query should only return records where there is an actual matching c record. So the original query is the same as if there were no such (+) suffixes.
Your query joins TEST_TABLE3 twice, while the first query only joins it once, and there are two conditions that determine how it is joined there. You should not split those conditions over two separate joins.
BTW, it is surprising that the SQL Fiddle site does not show an error, as it makes no sense to use the same alias twice. See for example how MySQL returns the error with the same query on dbfiddle (on all available versions of MySQL):
Not unique table/alias: 'C'
So to get the same result using the standard join notation, all joins should be inner joins:
SELECT *
FROM TEST_TABLE1 A
INNER JOIN TEST_TABLE2 B
ON A.COL11 = B.COL11
INNER JOIN TEST_TABLE3 C
ON A.COL11 = B.COL11
AND B.COL12 = C.COL12;
#tricot correctly pointed out that it's strange to have 2 aliases with the same name and not getting an error. Also, to answer your question :
In the first query, we are firstly performing cross join between all the 3 tables by specifying all the table names. After that, we are filtering the rows using the condition specified in the WHERE clause on output that we got after performing cross join.
In second query, you need to join test_table3 only once. Since now you have all the required aliases A,B,C as in the first query so you can specify 2 conditions after the last join as below:
SELECT A.* FROM TEST_TABLE1 A
LEFT JOIN TEST_TABLE2 B
ON A.COL11 = B.COL11
left join TEST_TABLE3 C
on B.COL12 =C.COL12 AND A.COL3 = C.COL13;
Related
I have a dilema, i had a teacher that thought me basically that inner joins are hell (he reproved me because I missed the delivery of the final proyect by 3 mins...), now i have another that tells me that using just selects is inefficient, so I don't know what is white nor black... could someone enlighten me with their knowledge?
Joins
SELECT
NombreP AS Nombre,
Nota
FROM Lleva
INNER JOIN Estudiante ON CedEstudiante = Estudiante.Cedula
WHERE
Lleva.SiglaCurso='CI1312';
No Joins
SELECT
NombreP AS Nombre,
Nota
FROM (
SELECT
Nota,
CedEstudiante
FROM Lleva
WHERE
SiglaCurso='CI1312'
) AS Lleva, (
SELECT
NombreP,
Cedula
FROM Estudiante
) AS Estudiante
WHERE
CedEstudiante = Estudiante.Cedula;
So wich one is more efficient?
Lets re-write the code so it's easier to understand:
SELECT E.NombreP AS Nombre
,L.Nota
FROM Lleva L INNER JOIN Estudiante E ON L.CedEstudiante = E.Cedula
WHERE
L.SiglaCurso='CI1312';
A table using a subquery may look like this:
SELECT L.Nota
,(SELECT E.Nombre
FROM Estudiante E
WHERE E.Cedula = L.CedEstudiante
)
FROM Lleva L
WHERE
L.SiglaCurso='CI1312'
What you actually did in your original query was an implicit join. This is similar to inner join, without declaring the exact joining conditions. Implicit joins will attempt to join on similarly named columns between tables. Most programmers do not advise or use implicit join.
As for join versus subquery, they are applied in different situations.
They are not equivalent. Notice what I have put in bold below:
A subquery will guarantee 1 returned value or return NULL; if there are multiple values returned in the subquery you will get an error for returning more than 1 value and have to solve the problem with an aggregation perhaps (max value, top 1 value). Subqueries that return no match will return NULL without affecting the rest of the row.
The JOIN (INNER JOIN) operates differently. It can match rows and get the single value you're looking for just like a subquery. But a join can multiply returned rows if the joining conditions are not distinct (singular/non-repeating). This is why joins are usually done on Primary Keys (PK's). PK's are distinct by definition. The INNER JOIN will also remove rows if a joining condition doesn't occur between tables. This may be what your first professor was trying to explain- an INNER JOIN can work in many cases - similar to a subquery- but can also can return additional rows or remove rows from the output.
This question already has answers here:
What is the difference between "INNER JOIN" and "OUTER JOIN"?
(28 answers)
Closed 8 years ago.
I have about 6 months novice experience in SQL, TSQL, SSIS, ETL. As I find myself using JOIN statements more and more in my intern project I have been experimenting with the different JOIN statements. I wanted to confirm my findings. Are the following statements accurate pertaining to the conclusion of JOIN statements in SQL Server?:
1)I did a LEFT OUTER JOIN query and did the same query using JOIN which yielded the same results; are all JOIN statements LEFT OUTER associated in SQL Server?
2)I did a LEFT OUTER JOIN WHERE 2nd table PK (joined to) IS NOT NULL and did the same query using an INNER JOIN which yielded the same results; is it safe to say the the INNER JOIN statement will yield only matched records? and is the same as LEFT OUTER JOIN where joined records IS NOT NULL ?
The reason I'm asking is because I have been only using LEFT OUTER JOINS because that is what I was comfortable with. However, I want to eliminate as much code as possible when writing queries to be more efficient. I just wanted to make sure my observations are correct.
Also, are there any tips that you could provide on easily figuring out which JOIN statement is appropriate for specific queries? For instance, what JOIN would you use if you wanted to yield non-matching records?
Thanks.
A join or inner join (same thing) between table A and table B on, for instance, field1, would narrow in on all rows of table A and B sharing the same field1 value.
A left outer join between A and B, on field1, would show all rows of table A, and only those rows of table B that have a field1 existing in table A.
Where the rows of field1 on table A have a field1 value that doesn't exist in table B, the table B value would show null for field1, but the row of table A would be retained because it is an outer join. These are rows that wouldn't show up in a join which is an implied inner join.
If you get the same results doing a join between table A and table B as you do a left outer join between table A and B, then whatever fields you're joining on have values that exist in both tables. No value for any of the joined fields in A or B exist exclusively in A or B, they all exist in both A and B.
It is also possible you're putting criteria into the where clause that belongs in the on clause of the outer join, which may be causing your confusion. In my example above of tables A and B, where A is being left outer joined with B, you would put any criteria related to table B in the on clause, not the where clause, otherwise you would essentially be turning the outer join into an inner join. For example if you had b.field4 = 12 in the WHERE clause, and table B didn't have a match with A, it would be null and that criteria would fail, and it'd no longer come back even though you used a left outer join. That may be what you are referring to.
JOIN's are mapped to 'INNER JOIN' by default
This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
Is it better to do an equi join in the from clause or where clause
I have following two SQL queries. Can you please explain me the differences between these two ?
QUERY: 1
SELECT a.*, b.*
FROM Table1 a
INNER JOIN Table2 b
ON a.id = b.id
AND a.col = 'value'
QUERY: 2
SELECT a.*, b.*
FROM Table1 a
INNER JOIN Table2 b
ON a.id = b.id
WHERE a.col = 'value'
Thanks
Nothing unless case sensitivity is turned on on the database
For an INNER JOIN, filtering on the JOIN condition vs filtering in the WHERE clause should give identical results.
If this were an OUTER JOIN, the results would be different since the first one pre-filters the results in the source table before the JOIN condition.
in this particular case it's the same result, but it's not the same thing.
the WHERE applies to the whole select, while the ON ... AND is only for the inner join (actually probably the same thing, but not for outer joins)
Yes, everything is pretty much the same. Case sensitive database will return error if your ID fields is not spelled correctly.
The difference is purely stylistic. Personally, I like to put the clauses that are key to performing the table joins up under the 'JOIN ... ON a=b' section, and put the clauses that have to do with my specific query filtering under the "WHERE" clause. There is no technical reason to do it one way or the other though, so long as you're talking about inner joins.
This question already has answers here:
SQL left join vs multiple tables on FROM line?
(12 answers)
Closed 8 years ago.
I'm curious as to why we need to use LEFT JOIN since we can use commas to select multiple tables.
What are the differences between LEFT JOIN and using commas to select multiple tables.
Which one is faster?
Here is my code:
SELECT mw.*,
nvs.*
FROM mst_words mw
LEFT JOIN (SELECT no as nonvs,
owner,
owner_no,
vocab_no,
correct
FROM vocab_stats
WHERE owner = 1111) AS nvs ON mw.no = nvs.vocab_no
WHERE (nvs.correct > 0 )
AND mw.level = 1
...and:
SELECT *
FROM vocab_stats vs,
mst_words mw
WHERE mw.no = vs.vocab_no
AND vs.correct > 0
AND mw.level = 1
AND vs.owner = 1111
First of all, to be completely equivalent, the first query should have been written
SELECT mw.*,
nvs.*
FROM mst_words mw
LEFT JOIN (SELECT *
FROM vocab_stats
WHERE owner = 1111) AS nvs ON mw.no = nvs.vocab_no
WHERE (nvs.correct > 0 )
AND mw.level = 1
So that mw.* and nvs.* together produce the same set as the 2nd query's singular *. The query as you have written can use an INNER JOIN, since it includes a filter on nvs.correct.
The general form
TABLEA LEFT JOIN TABLEB ON <CONDITION>
attempts to find TableB records based on the condition. If the fails, the results from TABLEA are kept, with all the columns from TableB set to NULL. In contrast
TABLEA INNER JOIN TABLEB ON <CONDITION>
also attempts to find TableB records based on the condition. However, when fails, the particular record from TableA is removed from the output result set.
The ANSI standard for CROSS JOIN produces a Cartesian product between the two tables.
TABLEA CROSS JOIN TABLEB
-- # or in older syntax, simply using commas
TABLEA, TABLEB
The intention of the syntax is that EACH row in TABLEA is joined to EACH row in TABLEB. So 4 rows in A and 3 rows in B produces 12 rows of output. When paired with conditions in the WHERE clause, it sometimes produces the same behaviour of the INNER JOIN, since they express the same thing (condition between A and B => keep or not). However, it is a lot clearer when reading as to the intention when you use INNER JOIN instead of commas.
Performance-wise, most DBMS will process a LEFT join faster than an INNER JOIN. The comma notation can cause database systems to misinterpret the intention and produce a bad query plan - so another plus for SQL92 notation.
Why do we need LEFT JOIN? If the explanation of LEFT JOIN above is still not enough (keep records in A without matches in B), then consider that to achieve the same, you would need a complex UNION between two sets using the old comma-notation to achieve the same effect. But as previously stated, this doesn't apply to your example, which is really an INNER JOIN hiding behind a LEFT JOIN.
Notes:
The RIGHT JOIN is the same as LEFT, except that it starts with TABLEB (right side) instead of A.
RIGHT and LEFT JOINS are both OUTER joins. The word OUTER is optional, i.e. it can be written as LEFT OUTER JOIN.
The third type of OUTER join is FULL OUTER join, but that is not discussed here.
Separating the JOIN from the WHERE makes it easy to read, as the join logic cannot be confused with the WHERE conditions. It will also generally be faster as the server will not need to conduct two separate queries and combine the results.
The two examples you've given are not really equivalent, as you have included a sub-query in the first example. This is a better example:
SELECT vs.*, mw.*
FROM vocab_stats vs, mst_words mw
LEFT JOIN vocab_stats vs ON mw.no = vs.vocab_no
WHERE vs.correct > 0
AND mw.level = 1
AND vs.owner = 1111
I have a query that shows me a listing of ALL opportunities in one query
I have a query that shows me a listing of EXCLUSION opportunities, ones we want to eliminate from the results
I need to produce a query that will take everything from the first query minus the second query...
SELECT DISTINCT qryMissedOpportunity_ALL_Clients.*
FROM qryMissedOpportunity_ALL_Clients INNER JOIN qryMissedOpportunity_Exclusions ON
([qryMissedOpportunity_ALL_Clients].[ClientID] <> [qryMissedOpportunity_Exclusions].[ClientID])
AND
([qryMissedOpportunity_Exclusions].[ClientID] <> [qryMissedOpportunity_Exclusions].[BillingCode])
The initial query works as intended and exclusions successfully lists all the hits, but I get the full listing when I query with the above which is obviously wrong. Any tips would be appreciated.
EDIT - Two originating queries
qryMissedOpportunity_ALL_Clients (1)
SELECT MissedOpportunities.MOID, PriceList.BillingCode, Client.ClientID, Client.ClientName, PriceList.WorkDescription, PriceList.UnitOfWork, MissedOpportunities.Qty, PriceList.CostPerUnit AS Our_PriceList_Cost, ([MissedOpportunities].[Qty]*[PriceList].[CostPerUnit]) AS At_Cost, MissedOpportunities.fBegin
FROM PriceList INNER JOIN (Client INNER JOIN MissedOpportunities ON Client.ClientID = MissedOpportunities.ClientID) ON PriceList.BillingCode = MissedOpportunities.BillingCode
WHERE (((MissedOpportunities.fBegin)=#10/1/2009#));
qryMissedOpportunity_Exclusions
SELECT qryMissedOpportunity_ALL_Clients.*, MissedOpportunity_Exclusions.Exclusion, MissedOpportunity_Exclusions.Comments
FROM qryMissedOpportunity_ALL_Clients INNER JOIN MissedOpportunity_Exclusions ON (qryMissedOpportunity_ALL_Clients.BillingCode = MissedOpportunity_Exclusions.BillingCode) AND (qryMissedOpportunity_ALL_Clients.ClientID = MissedOpportunity_Exclusions.ClientID)
WHERE (((MissedOpportunity_Exclusions.Exclusion)=True));
One group needs to see everything, the other needs to see things they havn't deamed as "valid" missed opportunity as in, we've seen it, verified why its there and don't need to bother critiquing it every single month.
Generally you can exclude a table by doing a left join and comparing against null:
SELECT t1.* FROM t1 LEFT JOIN t2 on t1.id = t2.id where t2.id is null;
Should be pretty easy to adopt this to your situation.
Looking at your query rewritten to use table aliases so I can read it...
SELECT DISTINCT c.*
FROM qryMissedOpportunity_ALL_Clients c
JOIN qryMissedOpportunity_Exclusions e
ON c.ClientID <> e.ClientID
AND e.ClientID <> e.BillingCode
This query will produce a cartesian product of sorts... each and every row in qryMissedOpportunity_ALL_Clients will match and join with every row in qryMissedOpportunity_Exclusions where ClientIDs do not match... Is this what you want?? Generally join conditions are based on a column in one table being equal to the value of a column in the other table... Joining where they are not equal is unusual ...
Second, the second iniquality in the join conditions is between columns in the same table (qryMissedOpportunity_Exclusions table) Are you sure this is what you want? If it is, it is not a join condition, it is a Where clause condition...
Second, your question mentions two queries, but there is only the one query (above) in yr question. Where is the second one?