We Keep Coding

Join more than two tables with "not in" clause

Can you please show me where I can INNER JOIN the tables for the FK_roomTypesID, FK_BedTypeID and FK_roomViewsID? All of the FK are on different tables from the primary.
MySqlDataAdapter da = new MySqlDataAdapter("SELECT RoomNo, RoomBedsNo,
RoomSmoking, RoomMiniBar,RoomKitchen,RoomFirePlace,RoomBalcony, RoomVeranda,
RoomGarden, RoomEntrance, RoomAirCondition, RoomTV, FK_roomTypesID,
FK_BedTypeID, FK_roomViewsID FROM tblrooms WHERE roomsID NOT IN(SELECT FK_roomsID
FROM tblbooking WHERE '" + arrdate + "' < checkOutDate AND '" + depdate + "' >
checkInDate)", conn);

You have the following part in your query:
FROM tblrooms
You define your joins in your FROM clause, therefore you should modify the code above to something like:
FROM tblrooms
join tblroomtypes
on tblrooms.FK_roomTypesID = tblroomtypes.roomTypesID
join tblbedtypes
on tblrooms.FK_bedTypesID = tblbedtypes.bedTypesID
join tblroomviews
on tblrooms.FK_roomViewsID = tblroomviews.roomViewsID
Note, that you did not give too much information, so I am guessing your table and column names here. Also, note, that I did not select any new columns. Finally, note, that you need to resolve ambiguity if the problem is applicable to your case by referring to your columns as table's columns. And format your code, it is difficult to read it. I did not use the inner keyword for the joins, because join is inner by default.

Using a left join where is null in the update statement

I'm trying to use a left join to set the plot.jobs row to 0 when there isn't a match found in the booking table (the check is plot.plot_id = booking.plot_id).
However I keep getting the following error message:
An exception of type 'MySql.Data.MySqlClient.MySqlException' occurred
in MySql.Data.dll but was not handled in user code Additional
information: You have an error in your SQL syntax; check the manual
that corresponds to your MySQL server version for the right syntax to
use near 'FROM plot LEFT JOIN booking ON plot.plot_id =
booking.plot_id WHERE plot.plot_id' at line 1.
I was following this template here:
UPDATE <Table2>
SET Count = 0
FROM <Table2>
LEFT JOIN <Table1>
ON <Table2>.<JoinColumn> = <Table1>.<JoinColumn>
WHERE <Table1>.<JoinColumn> IS NULL
This is my code so far:
query = "UPDATE plot SET jobs = #jobCount WHERE plot_id = #plotID AND postcode='MK';";
query += "UPDATE plot " +
"SET jobs = 0 " +
"FROM plot " +
"LEFT JOIN booking " +
"ON plot.plot_id = booking.plot_id " +
"WHERE plot.plot_id IS NULL";
cmd = new MySqlCommand(query, _connection);

You are attempting to use the SQL Server syntax for UPDATE statements on MySQL. This doesn't work.
The UPDATE-syntax of MySQL is:
UPDATE [LOW_PRIORITY] [IGNORE] table_reference
SET col_name1={expr1|DEFAULT} [, col_name2={expr2|DEFAULT}] ...
[WHERE where_condition]
[ORDER BY ...]
[LIMIT row_count]
Where table_reference is defined with the JOIN-syntax.
Your query should be something like:
query += "UPDATE plot " +
"LEFT JOIN booking " +
"ON plot.plot_id = booking.plot_id " +
"SET jobs = 0 " +
"WHERE plot.plot_id IS NULL";
I am wondering if you don't mean: WHERE booking.plot_id IS NULL

Your syntax isn't quite right for MySQL. Do your LEFT JOIN before SET
Something like this should work (Note: this hasn't been tested).
query += "UPDATE plot p" +
"LEFT JOIN booking b" +
"ON p.plot_id = b.plot_id " +
"SET jobs = 0 " +
"WHERE p.plot_id IS NULL";

You are attempting to update the plot table setting jobs equal to zero when plot_id is null? What is the point of the join in this case? You are attempting to join records based on a null value which seems highly invalid, and you're not using the joined table booking for anything. I recommend removing the join and the from statement is not valid in an update as Saggio mentioned.
query = "UPDATE plot SET jobs = #jobCount WHERE plot_id = #plotID AND postcode='MK';";
query +="UPDATE plot " +
"SET jobs = 0 " +
"LEFT JOIN booking ON plot.plot_id = booking.plot_id" +
"WHERE booking.plot_id IS NULL;";
cmd = new MySqlCommand(query, _connection);

What is the difference between using a parameter and just putting a variable then convert it

So I'm retrieving data from the database using odbc. One of my fields, which was causing the problem, is AutoNumber. In my query when I use, '" convert.toint64(empid)"', I get an error of data type missmatch. But when I used parameter, it worked perfectly. Can someone tell me what is the difference. As far as I know, AutoNumber is a long integer and long int is 64, right?
here are both of my queries:
string cmdText = "SELECT p.projID, p.projName, a.wageperday " +
"FROM ((projects p INNER JOIN assigns a ON p.projID = a.projname) " +
"INNER JOIN empos e ON a.employeeID= e.ID) " +
"WHERE a.employeeID = ?";
OdbcCommand assignslist = new OdbcCommand(cmdText, _connection);
assignslist.Parameters.AddWithValue("#empID", empid);
OdbcDataReader readassigns = assignslist.ExecuteReader();
GridView1.DataSource = readassigns;
GridView1.DataBind();
(this one works fine)
string cmdText = "SELECT p.projID, p.projName, a.wageperday " +
"FROM ((projects p INNER JOIN assigns a ON p.projID = a.projname) " +
"INNER JOIN empos e ON a.employeeID= e.ID) " +
"WHERE a.employeeID = '" + convert.toint64( empid ) + "';
OdbcCommand assignslist = new OdbcCommand(cmdText, _connection);
OdbcDataReader readassigns = assignslist.ExecuteReader();
GridView1.DataSource = readassigns;
GridView1.DataBind();
for this one, i get an error even if I removed the conversion, I get an error.
One more question, I don't understand INNER JOIN perfectly and the above code was from a user here who helped me. I don't understand why did he use "FROM ((projects p INNER JOIN assigns a ON p.projID = a.projname) " +
"INNER JOIN empos e ON a.employeeID= e.ID)"
two brackets -- I mean a bracket inside a bracket. And if I wanna join a 4th or 5th table do I have to put it inside a bracket aside from the main bracket? An example would be much appreciated!

If a.employeeID is a number then your problem are the quotes and the concatenation of a number to a string.
"WHERE a.employeeID = " + convert.toint64(empid).ToString()
but this make no sense because you have a string to build so, you could simply write
"WHERE a.employeeID = " + empid.ToString();
however use always the parameterized query. That is the correct way to go.
A parameterized query allows the Framework code to pass the parameters with the correct format and you don't have to worry about quoting values, format of dates and decimal separators. (And last but not least, you avoid any possibilities of Sql Injection)
For the second part of your question, JOIN is used to put togheter data from two tables, when you have more than one JOIN the parenthesys help to understand how the grouping from the tables are perfomed. First the data from projects and assigns are grouped together following the rules of the INNER JOIN then the resulting set of data is joined with the data from the employee table following the rules of the second join.

In your second query, instead of using convert.toint64( empid ) try using empid.ToString():
"WHERE a.employeeID = " + empid.ToString();
The error you were receiving was due to the fact that you were trying to concatenate a string with an integer.
With regards to the INNER JOINs you are using, the use of the brackets is dependant upon the database you are using. In most cases, you will not need the brackets at all and they can be removed without any issue, so you could rewrite the query to:
string cmdText = "SELECT p.projID, p.projName, a.wageperday " +
"FROM projects p " +
"INNER JOIN assigns a ON p.projID = a.projname " +
"INNER JOIN empos e ON a.employeeID = e.ID " +
"WHERE a.employeeID = " empid.ToString();

You're trying to add a string and an integer, which isn't allowed automatically. You'd have to convert the number to a string first, like this:
"WHERE a.employeeID = '" + empid.ToString() + "';
But, using parameters is the better way for other reasons (best habit to be in, to avoid SQL-injection attacks, etc.).

Trying to Query 2 Access tables with 1 statement

I've been doing research and cannot find a way to get data from multiple tables using one query. What I want to achieve is getting information from one table in an Access database and using that information to get more data from a different table in the same database
Here's the code I have so far...
string end = "ENDDATE";
string qual = "CGA0113";
string start = "START";
ad.SelectCommand = new OleDbCommand("SELECT* FROM [AC_ECONOMIC] Where KEYWORD = '" + end + "'AND QUALIFIER = '" + qual + "' AND EXPRESSION LIKE 'SN%'", con);
I want to be able to get data from a separate table called AC_PROPERTY with the same SN from the query above from that AC_ECONOMIC table. I was researching joins and believe that is the way to go but I'm not quite sure of the syntax or if that would even work. Any help would be greatly appreciated.

Correct, an inner join would be the way to go. You basically need to construct a statement that joins the 2 tables by their common field. Something like this:
SELECT * FROM [AC_ECONOMIC] a
INNER JOIN [AC_PROPERTY] b
on [AC_ECONOMIC].SN=[AC_PROPERTY].SN
Where KEYWORD = '" + end + "'AND QUALIFIER = '" + qual + "' AND EXPRESSION LIKE 'SN%'
Above statement is just an example. I don't know if the common column between the 2 tables is actually called "SN".
UPDATE:
As suggested by #Geek, you can also alias the tables after you reference them in your statement, this saves you from typing the full table names every time. For example, the above statement can also be written as:
SELECT a.*, b.* FROM [AC_ECONOMIC] a
INNER JOIN [AC_PROPERTY] b
on a.SN=b.SN
Where a.KEYWORD = '" + end + "'AND a.QUALIFIER = '" + qual + "' AND b.EXPRESSION LIKE 'SN%'
Note how the WHERE clause is filtering using columns from both, "a" (AC_ECONOMIC) and b (AC_PROPERTY).

Updating only the rows matched by field value from CROSS JOIN of 2 tables (SQL)

Here's what needs to be done:
Match 2 fields from different tables in SQL Server( done ), update fields in table1 where table1.field1 = table2.field2 (problem, it just updates all the records in the table)
Here's what I have , where tempTableName is a table imported to SQL Server for the purpose of the query:
"UPDATE Table1 SET Table1.fieldN ='" + DateTime.Now.DayOfYear + "' FROM " + tempTableName + " CROSS JOIN Table1 WHERE (" + tempTableName + ".fieldX = Table1.fieldY)"
Here's what I've figured out:
everything after the FROM is useless as far as actual functionality is concerned, it runs the query but the results are not "linked" in any way to the actual UPDATE statement
To sum up :
The query I've figured out updates all records in table1, I need the query to update only the rows matched by the query after FROM
PS. Forgive me if this seems trivial and that I haven't done research but the fact of the matter is that 2 weeks ago I'd never even heard of SQL and have relied heavily on SO for direction and advice.

Your CROSS JOIN should be an INNER JOIN instead.
"UPDATE t1
SET fieldN ='" + DateTime.Now.DayOfYear + "'
FROM " + tempTableName + " t2 INNER JOIN Table1 t1 ON t1.fieldy = t2.fieldx"