I have four tables I want to join and get data from. The tables look something like...
Employees (EmployeeID, GroupID[fk], EmployeeName, PhoneNum)
Positions (PositionID, PositionName)
EmployeePositions (EployeePositionID, EmployeeID[fk], PositionID[fk])
EmployeeGroup (GroupID, GroupName)
[fk] = foreign key
I want to create a query that will return all the information about an employee(given by EmployeeID). I want a query that will return the given employees Name, position(s), and group in one row.
I think it needs to involve joins, but I am not sure how to format the queries. MYSQL's manual is technical beyond my comprehension. I would be very grateful for any help.
It seems you have trouble with SQL, in general, rather than with mySQL in particular. The documentation of mySQL provides details about the various SQL expressions, but generally assume some familiarity with SQL. To get a quick start on SQL you may consider this W3schools.com primer.
The query you need is the following.
SELECT EmployeeName, PositionName, GroupName
FROM Employees E
LEFT JOIN EmployeePositions EP ON EP.EmployeeID = E.EmployeeID
LEFT JOIN Positions P ON P.PositionID = EP.PositionId
LEFT JOIN EmployeeGroup EG ON EG.GroupId = E.GroupId
WHERE E.EmployeeId = some_value
A few things to note:
The 'LEFT' in 'LEFT JOIN' will result in producing NULL in lieu of PositionName or GroupName when the corresponding tables do not have a value for the given FK. (Should only happen if the data is broken, say if for example some employees have GroupId 123 but somehow this groupid was deleted from the EmployeeGroup table.
The query returns one line per employee (1). You could use an alternative search criteria, for example WHERE EmployeeName = 'SMITH', and get a listing of all employees with that name. Indeed without a WHERE clause, you'd get a list of all employees found in Employees table.
(1) that is assuming that each employee can only have one position. If somehow some employees have more than one position (i.e. multiple rows in EmployeePositions for a given EmployeeID), you'd get several rows per employee, the Name and Group being repeated and a distinct PostionName.
Edit:
If a given employee can have multiple positions, you can use the query suggested by Tor Valamo, which uses a GROUP BY construct, with GROUP_CONCAT() to pivot all the possible positions in one single field value in the returned row.
SELECT e.EmployeeID, e.EmployeeName, e.PhoneNum,
g.GroupName, GROUP_CONCAT(p.PositionName) AS Positions
FROM Employees e
LEFT JOIN EmployeeGroup g ON g.GroupID = e.GroupID
LEFT JOIN EmployeePositions ep ON ep.EmployeeID = e.EmployeeID
LEFT JOIN Positions p ON p.PositionID = ep.PositionID
WHERE e.EmployeeID = 1
GROUP BY e.EmployeeID
Returns positions in a comma separated string on one row.
Related
I have two tables and I want to join them to get the desired output.
Say the 1st table (seat1) is
and the 2nd table (collegestudents) is
The desired output is
I have tried the below code. But it fails to give the desired result.
$rde2=mysqli_query($con, "select * from seat1 s
left JOIN collegestudents c ON c.Roll = s.Roll
");
Any help please.
You want a left join. Your query looks fine, but you would need not to use select *, and instead explictly list the columns that you want to select, using table prefixes. Otherwise, since you have a Roll column in both tables, a name clashes will happen, that your application apparently does not handle well.
select
s.Roll,
c.Name,
s.Subject
from seat1 s
left join collegestudents c on c.Roll = s.Roll
I have several different tables in my database(mySQL).
Here are the relevant coumns for the tables
table_tournaments
tournamentId
tournamnetName
tournamentStatus
table_tournament_results
tournamentId
playerId
playerName
playerRank
tournamnetParticipants
table_players
playerId
playerName
The tournaments table contains the information about the tournament, the tournament results table shows the results from that table
I want to search the tournaments table by name and then with the returned results get the information from the tournament results table.
SELECT * FROM `tournaments` `WHERE` `tournamentName` LIKE "%Query%"
I'm not sure how to go about this, maybe I need to do something via PHP, any and all help is appreciated.
You can get the results you want with a join operation.
This is an example of an outer join, returning all rows from t that have the string 'foo' appearing as part of tournament_name, along with any matching rows from r.
A relationship between rows in the two tables is established by storing a common value in the tournamentId column of the two tables. The predicate in the ON clause specifies the condition that determines if a row "matches".
SELECT t.tournamentId
, t.tournamentName
, t.tournamentStatus
, r.playerId
, r.playerName
, r.playerRank
FROM table_tournaments t
LEFT
JOIN table_tournament_results r
ON r.tournamentId = t.tournamentId
WHERE t.tournament_name LIKE '%foo%'
ORDER
BY t.tournamentId
, r.playerId
The t and r that appear after the table names are table aliases, we can qualify references to the columns in each table by prefacing the column name with the table alias and a dot. This makes the column reference unambiguous. (In the case of tournamentId, MySQL doesn't know if you are referring to the column in t or r, so we qualify it to make it explicit. We follow this same pattern for all column references. Then, someone reading the statement doesn't need to wonder which table contains the column playerId.
Your Query may be like this
SELECT a.*, b.tournamnetName FROM table_tournament_results a
left join table_tournaments on a.tournamentId=b.tournamentId
WHERE b.tournamnetName LIKE "%Query%"
I'm creating a website where the user can add some information about multiple computers into a database.
In the mysql database I have 3 tables. TypeOfMachine, Software and Hardware. The only common thing they have is the NumberOfMachine.
I must create a page where the user can run reports showing devices that have a specific piece of software installed (user specified) or specific hardware (user specified) after he submitted the computer's info.
Any ideas how I can do this?
And how I can connect all 3 tables into one?
I thought about this code but i dont know what to put in WHERE. I have 10 variables. and I have to show all the computers with what the user has asked and their info as well!
$search1 = "
SELECT
TypeOfMachine.NumberOfMachine, TypeOfMachine.TypeOfMachine, TypeOfMachine.OS, Software.Software1, Software.Software2, Software.Software3, Hardware.SSD, Hardware.Harddisk, Hardware.MonitorSize, Hardware.Ram, Hardware.Rom, Hardware.Processor
FROM TypeOfMachine, Software, Hardware
WHERE
but i
You want to use a join. This example is based on the fact that you've said the NumberOfMachine field is present in all tables and is a common link between them:
SELECT
TypeOfMachine.NumberOfMachine,
TypeOfMachine.TypeOfMachine,
TypeOfMachine.OS,
Software.Software1,
Software.Software2,
Software.Software3,
Hardware.SSD,
Hardware.Harddisk,
Hardware.MonitorSize,
Hardware.Ram,
Hardware.Rom,
Hardware.Processor
FROM TypeOfMachine
LEFT JOIN Software
ON Software.NumberOfMachine = TypeOfMachine.NumberOfMachine
LEFT JOIN Hardware
ON Hardware.NumberOfMachine = TypeOfMachine.NumberOfMachine
WHERE
...
It's general question, I don't know which tables contains a spesific columns as indicator for all tables. It's about inner and outer join:
The two common types of joins are an inner join and an outer join. The difference between an inner and outer join is in the number of rows included in the results table.
Inner join: The results table produced by an inner join contains only rows that existed in both tables.
Outer join: The combined table produced by an outer join contains all rows that existed in one table with blanks in the columns for the rows that did not exist in the second table.
For instance, if table1 contains a row for Joe and a row for Sally, and table2 contains only a row for Sally, an inner join would contain only one row: the row for Sally. However, an outer join would contain two rows — a row for Joe and a row for Sally — even though the row for Joe would have a blank field for weight.
The results table for the outer join contains all the rows for one table. If any of the rows for that table don’t exist in the second table, the columns for the second table are empty. Clearly, the contents of the results table are determined by which table contributes all its rows, requiring the second table to match it.
Two kinds of outer joins control which table sets the rows and which must match: a LEFT JOIN and a RIGHT JOIN.
You use different SELECT queries for an inner join and the two types of outer joins. The following query is an inner join:
SELECT columnnamelist FROM table1,table2
WHERE table1.col2 = table2.col2
And these queries are outer joins:
SELECT columnnamelist FROM table1 LEFT JOIN table2
ON table1.col1=table2.col2
SELECT columnnamelist FROM table1 RIGHT JOIN table2
ON table1.col1=table2.col2
In all three queries, table1 and table2 are the tables to be joined. You can join more than two tables. In both queries, col1 and col2 are the names of the columns being matched to join the tables. The tables are matched based on the data in these columns. These two columns can have the same name or different names, but they must contain the same type of data.
For general concept you can use #Scrowler suggestion, or this one:
http://stackoverflow.com/questions/1204217/mysql-select-join-3-tables
I need to report the number of records that match each of several criteria. For example, how many customers live in each state. I know I can accomplish this through a series of MySQL SELECT statements, but that seems cumbersome and produces a ton of (unnecessary?) MySQL calls.
Can you tell me a better method to use? Can I query the database with one SELECT statement and then use PHP to filter the results to variables?
I'd suggest creating a view for this task just to hide the complexity of the query. Also, in the event that your table schema changes, it is likely that you are still going to want to retrieve this same information from the database. You'd be able to change the view in one place, instead of having to change the queries in, possibly, multiple places to satisfy your schema changes.
I'll just show you the queries, though, since you'd need to know how to do that to create a view anyways.
Sticking with your example of customers living in each state, let's pretend you also want statistics on how many customers share the same last name.
I've setup a mock structure of what your database might be like at this SqlFiddle.
Customers with Same LastName
The following query might be used to get the number of customers with the same last name:
SELECT
LastName AS "Value",
COUNT(*) AS "Count"
FROM Customers
GROUP BY
LastName;
Customers in Same State
Similarly, the customers in the same state might be retrieved with a query as follows:
SELECT
S.Name AS "Value",
COUNT(*) AS "Count"
FROM Customers AS C
INNER JOIN CustomerAddresses AS CA ON C.Id = CA.CustomerId
INNER JOIN Addresses AS A ON CA.AddressId = A.Id
INNER JOIN States AS S ON A.State = S.Id
GROUP BY
A.State;
Getting Your Desired Format
The format that you want is an aggregation of these two queries. You want both returned as a single result set. So, let's workout a schema for the returned table:
ResultType - This will hold a value that corresponds to the type of result. i.e. "State"
Value - This will hold the value of the aggregated column. i.e. "Florida"
Count - This will hold the total number of records that match the aggregated column.
So, now that we have a format, let's create a query that uses our two queries from above, and puts them into this format.
First, I add a new field to each of the above queries: ResultType
For example:
"LastName" AS "ResultType"
Now, I combine my queries into a single query using the UNION ALL statement:
SELECT * FROM (
/* LastName query */
SELECT
"LastName" AS "ResultType",
LastName AS "Value",
COUNT(*) AS "Count"
FROM Customers
GROUP BY
LastName
UNION ALL
/* States query */
SELECT
"State" AS "ResultType",
S.Name AS "Value",
COUNT(*) AS "Count"
FROM Customers AS C
INNER JOIN CustomerAddresses AS CA ON C.Id = CA.CustomerId
INNER JOIN Addresses AS A ON CA.AddressId = A.Id
INNER JOIN States AS S ON A.State = S.Id
GROUP BY
A.State
) AS A
In my SqlFiddle above, this produces an output like:
RESULTTYPE VALUE COUNT
=================================
LastName Jetson 1
LastName Johnson 2
LastName Milton 1
State Florida 2
State Georgia 1
State Utah 1
As you can see, this could get quite complex, so you might consider looking into placing this into a view. Then, you'd be able to query your view, as if it was the table above (ResultType, Value, and Count). That would also allow you to filter on it.
create select query make number of aliens of table and make your related columns aliens which is you want to use.
lest see sample example
SELECT a.id AS id
FROM Table1 AS a
WHERE ...
UNION ALL
SELECT b.name AS name
FROM Table2 AS b
WHERE ...
ORDER BY (a or b) ...
Can you let me know if my interpretation is correct (the last AND part)?
$q = "SELECT title,name,company,address1,address2
FROM registrations
WHERE title != 0 AND id IN (
SELECT registrar_id
FROM registrations_industry
WHERE industry_id = '$industryid'
)";
Below was really where I am not sure:
... AND id IN (select registrar_id from registrations_industry where industry_id='$industryid')
Interpretation: Get any match on id(registrations id field) equals registrar_id(field) from the join table registrations_industry where industry_id equals the set $industryid
Is this select statement considered a sub routine since it's a query within the main query?
So an example would be with the register table id search to 23 would look like:
registrations(table)
id=23,title=owner,name=mike,company=nono,address1=1234 s walker lane,address2
registrations_industry(table)
id=256, registrar_id=23, industry_id=400<br>
id=159, registrar_id=23, industry_id=284<br>
id=227, registrar_id=23, industry_id=357
I assume this would return 3 records with the same registration table data And of course varying registrations_industry returns.
For a given test data set your query will return one record. This one:
id=23,title=owner,name=mike,company=nono,address1=1234 s walker lane,address2
To get three records with the same registration table data and varying registrations_industry you need to use JOIN.
Something like this:
SELECT r.title, r.name, r.company, r.address1, r.address2
FROM registrations AS r
LEFT OUTER JOIN registrations_industry AS ri
ON ri.registrar_id=r.id
WHERE r.title!=0 AND ri.industry_id={$industry_id}
Sorry for the essay, I didn't realize it was as long as it is until looking at it now. And although you've checked an answer, I hope you read this gain some insight into why this solution is preferred and how it evolved out of your original query.
First things first
Your query
$q = "SELECT title,name,company,address1,address2
FROM registrations
WHERE title != 0 AND id IN (
SELECT registrar_id
FROM registrations_industry
WHERE industry_id = '$industryid'
)";
seems fine. The IN syntax is equivalent to a number of OR matches. For example
WHERE field_id IN (101,102,103,105)
is functionally equivalent to
WHERE (field_id = 101
OR field_id = 102
OR field_id = 103
OR field_id = 105)
You complicate it a bit by introducing a subquery, no problem. As long as your subquery returns one column (and yours does), passing it to IN will be fine.
In your case, you're comparing registrations.id to registrations_industry.registrar_id. (Note: This is just <table>.<field> syntax, nothing special, but helpful to disambiguate what tables your fields are in.)
This seems fine.
What happens
SQL would first run the subquery, generating a result set of registrar_ids where the industry_id was set as specified.
SQL would then run the outer query, replacing the subquery with its results and you would get rows from registrations where registrations.id matched one of the registrar_ids returned from the subquery.
Subqueries are helpful to debug your code, because you can pull out the subquery and run it separately, ensuring its output is as you expect.
Optimization
While subqueries are good for debugging, they're slow, at least slower than using optmized JOIN statements.
And in this case, you can convert your query to a single-level query (without subqueries) by using a JOIN.
First, you'd start with basically the exact same outer query:
SELECT title,name,company,address1,address2
FROM registrations
WHERE title != 0 AND ...
But you're also interested in data from the registrations_industry table, so you need to include that. Giving us
SELECT title,name,company,address1,address2
FROM registrations, registrations_industry
WHERE title != 0 AND ...
We need to fix the ... and now that we have the registrations_industry table we can:
SELECT title,name,company,address1,address2
FROM registrations, registrations_industry
WHERE title != 0
AND id = registrar_id
AND industry_id = '$industryid'
Now a problem might arise if both tables have an id column -- since just saying id is ambiguous. We can disambiguate this by using the <table>.<field> syntax. As in
SELECT registrations.title, registrations.name,
registrations.company, registrations.address1, registrations.address2
FROM registrations, registrations_industry
WHERE registrations.title != 0
AND registrations_industry.industry_id = '$industryid'
We didn't have to use this syntax for all the field references, but we chose to for clarity. The query now is unnecessarily complex because of all the table names. We can shorten them while still providing disambiguation and clarity. We do this by creating table aliases.
SELECT r.title, r.name, r.company, r.address1, r.address2
FROM registrations r, registrations_industry ri
WHERE r.title != 0
AND ri.industry_id = '$industryid'
By placing r and ri after the two tables in the FROM clause, we're able to refer to them using these shortcuts. This cleans up the query but still gives us the ability to clearly specify which tables the fields are coming from.
Sidenote: We could be more explicit about the table aliases by including the optional AS e.g. FROM registrationsASr rather than just FROM registrations r, but I typically reserve AS for field aliases.
If you run the query now you will get what is called a "Cartesian product" or in SQL lingo, a CROSS JOIN. This is because we didn't define any relationship between the two tables when, in fact, there is one. To fix this we need to reintroduce part of the original query that was lost: the relationship between the two tables
r.id = ri.registrar_id
so that our query now looks like
SELECT r.title, r.name, r.company, r.address1, r.address2
FROM registrations r, registrations_industry ri
WHERE r.title != 0
AND r.id = ri.registrar_id
AND ri.industry_id = '$industryid'
And this should work perfectly.
Nitpicking -- implicit vs. explicit joins
But the nitpicker in me needs to point out that this is called an "implicit join". Basically you're joining tables but not using the JOIN syntax.
A simpler example of an implicit join is
SELECT *
FROM foo f, bar b
WHERE f.id = b.foo_id
The corresponding explicit syntax is
SELECT *
FROM foo f
JOIN bar b ON f.id = b.foo_id
The result will be identical but it is using proper (and clearer) syntax. (Its clearer because it explicitly stats that there is a relationship between the foo and bar tables and it is defined by f.id = b.foo_id.)
We could similarly express your implicit query
SELECT r.title, r.name, r.company, r.address1, r.address2
FROM registrations r, registrations_industry ri
WHERE r.title != 0
AND r.id = ri.registrar_id
AND ri.industry_id = '$industryid'
explicitly as follows
SELECT r.title, r.name, r.company, r.address1, r.address2
FROM registrations r
JOIN registrations_industry ri ON r.id = ri.registrar_id
WHERE r.title != 0
AND ri.industry_id = '$industryid'
As you can see, the relationship between the tables is now in the JOIN clause, so that the WHERE and subsequent AND and OR clauses are free to express any restrictions. Another way to look at this is if you took out the WHERE + AND/OR clauses, the relationship between tables would still hold and the results would still "make sense" whereas if you used the implicit method and removed the WHERE + AND/OR clauses, your result set would contain rows that were misleading.
Lastly, the JOIN syntax by itself will cause rows that are in registrations, but do not have any corresponding rows in registrations_industry to not be returned.
Depending on your use case, you may want rows from registrations to appear in the results even if there are no corresponding entries in registrations_industry. To do this you would use what's called an OUTER JOIN. In this case, we want what is called a LEFT OUTER JOIN because we want all of the rows of the table on the left (registrations). We could have alternatively used RIGHT OUTER JOIN for the right table or simply OUTER JOIN for the outer join of both tables.
Therefore our query becomes
SELECT r.title, r.name, r.company, r.address1, r.address2
FROM registrations r
LEFT OUTER JOIN registrations_industry ri ON r.id = ri.registrar_id
WHERE r.title != 0
AND ri.industry_id = '$industryid'
And we're done.
The end result is we have a query that is
faster in terms of runtime
more compact / concise
more explicit about what tables the fields are coming from
more explicit about the relationship between the tables
A simpler version of this query would be:
SELECT title, name, company, address1, address2
FROM registrations, registrations_industry
WHERE title != 0
AND id = registrar_id
AND industry_id = '$industryid'
Your version was a subquery, this version is a simple join. Your assumptions about your query are generally correct, but it is harder for SQL to optimize and a little harder to unravel to anyone trying to read the code. Also, you won't be able to extract the data from the registrations_industry table in that parent SELECT statement because it's not technically joining and the subtable is not a part of the parent query.