Develop Reference

How to enforce a unique field with respect to other fields?

I wanted to make sure there were no duplicate timestamps in my table but then I found that it may be problematic if I simply use UNIQUE, because right now my table has:
_ID field (autoincrement)
Account ID (integer, links to an account table)
Category ID (integer, links to a category table)
Value (the value of this category for this account)
Timestamp (the timestamp of this value of this category for this account)
Is there a way to designate the timestamp field as unique within the context of the account ID and category ID? As in, it should not be possible to put in two values for a single timestamp, with respect to account and category. But the timestamp may show up multiple times in the table as a whole, either because it corresponds to other Categories and/or Accounts.

you can create a trigger and check whatever constraint you need:
#Override
public void onCreate(SQLiteDatabase database, ConnectionSource connectionSource) {
database.execSQL("CREATE TRIGGER UNIQUE_TIMESTAMP_TRIGGER " +
"AFTER INSERT ON MY_TABLE FOR EACH ROW " +
"BEGIN " +
" SELECT RAISE(ABORT, 'Timestamp not unique') WHERE " +
" EXISTS(SELECT TIMESTAMP MY_TABLE T2 " +
" WHERE T2.TIMESTAMP = NEW.TIMESTAMP AND T2.ACCOUNT_ID = NEW.ACCOUNT_ID) " +
"END; ");
}

How to get meta data information of a sql query

I am using postgres 9.1 and java code for jdbc.
I may use a order by clause in my sql query string
I just want to get the meta data information of the query to find whether the query has order by clause or not. If it has then how many fields has been specified in the order by clause.
Ex:
order by age
order by age, name
order by age asc, name desc
In these example I just want to retrieve the number of parameters that are specified in the order by clause and their column names.

If your are getting your query as string you could simply parse it.
i.e. To figure out that ORDER BY is there
"SELECT * FROM MyTable ORDER BY SomeColumn".toLowerCase().indexOf("order by") // if it's return -1 query does not contains order by section otherwise it returns start index for first occurence "ORDER BY" in given string
For more complex searching in string you may need to use RegExp

You can do it by breaking an SQL query into part and then reassigning.
Like
String sql="SELECT NAME,COMPANY,FNAME,AGE FROM COMP_DATA JOIN PERSONAL_DATA WHERE (1=1) AND FNAME='Vaibs' ORDER BY AGE";
While writing in JAVA do as below.
Break Whole query into String parts and recombine it like this.
String strSQL = "SELECT " + "NAME"+",COMPANY"+",FNAME"+",AGE" + "FROM "
+ getTableName1(); //getTableName1() return tablename
strSQL+="JOIN "+ getTable2()+"";//getTable2() return tablename as well
String strWhere = " WHERE (1=1) " + " and FNAME='" + fname+ "';
String orderBySQL = " Order by " + i_will_return_string_to_order_by();
//return AGE in our case
String FinalString= strSQL +strWhere +orderBySQL ;
SOP order by to get what you want.
Hope that helped.

Hibernate query building based on conditions

I have a form where user can select search criteria.
The criterias are say:
Product Name: Input field
Name Option: Radio button group - begins with (default selected)/ is/ contains
Country: dropdown of country
Status: All, Active, Blocked
Type: All, One, Two, Three
Only Product Name is mandatory. Other dropdowns are optional.
So if country is not given, I should find products for all countries.
If active is not given, I should find both active and blocked products.
If Type is not given, I should return all the three types products.
I am building hibernate query as below:
String productName = searchCriteria.getValue("productName");
String productNameCriteria = searchCriteria.getValue("productNameCriteria");
String country = searchCriteria.getValue("country");
String status = searchCriteria.getValue("status");
String type = searchCriteria.getValue("type");
Query prodQuery = null;
String prodSql = "select count(*) from Product p where";
// is
if (productNameCriteria.equalsIgnoreCase("IS")){
prodSql += "p.productName = '"+productName+"'";
}
// begins with
else if (productNameCriteria.equalsIgnoreCase("BEGINS WITH")){
prodSql += "p.productName = '"+productName+"%'";
}
// contains
else (productNameCriteria.equalsIgnoreCase("BEGINS WITH")){
prodSql += "p.productName = '%"+productName+"%'";
}
if(!country.equalsIgnoreCase("0")){
prodSql += " and p.country = '"+country+"'";
}
if(!status.equalsIgnoreCase("ALL")){
if(status.equalsIgnoreCase("active"))
prodSql += " and p.status = 'active'";
else
prodSql += " and p.status = 'blocked'";
}
if(!type.equalsIgnoreCase("ALL")){
if(type.equalsIgnoreCase("one"))
prodSql += " and p.type = 'one'";
else if(type.equalsIgnoreCase("two"))
prodSql += " and p.type = 'two'";
else
prodSql += " and p.type = 'three'";
}
prodQuery = this.em.createQuery(prodSql);
List<Object[]> results = prodQuery.getResultList();
Am I doing query building the right way ? Or is there any other efficient method ???
Thanks for reading!!

Try looking at Criteria Query
Criteria crit = sess.createCriteria(Product.class);
if (productNameCriteria.equalsIgnoreCase("IS"))
crit.add( Restrictions.eq("productName", productName);
else if (productNameCriteria.equalsIgnoreCase("BEGINS WITH"))
crit.add( Restrictions.like("productName", productName + "%")
// etc
If you absolutely must build a string query then you should be using a StringBuilder
StringBuilder sb = new StringBuilder();
sb.append("select count(*) from Product p where ");
if (productNameCriteria.equalsIgnoreCase("IS"))
sb.append("p.productName = '").append(productName).append("'");
// etc
String query = sb.toString();
Using a StringBuilder reduces the number of instances created at runtime.
You could also look into using query parameters, which would reduce some of the query complexity, though I don't know what the runtime query performance effects are.
"select count(*) from Product p where p.productName = :productName"
"select count(*) from Product p where p.productName = ?"
You can then use Query#setParameter (or one of the other variants like setString) to define the values in the query. This is also a much, much better way of building the query because it's going to automatically manage quoting and escaping of values you're receiving from the UI. Use query parameters and not string concatenation, regardless of how you build the query string.

Yes .It will work if you build the query dynamically in this way .But the code will become tedious and noisy as it involves string manipulating of the where-condition clause .
For this kind of query 's use case , which is a search that allows users to specify a range of different property values to be matched by the returned result set , using Query By Example(QBE) is more efficient and elegant.
The idea of QBE is that you provide an instance of the queried class with some properties initialized, and the query will returns the records with matching property values.
Reference
Example JavaDocs
YouTube Hibernate Tutorial - Projections and Query By Example

Using one query in another in JDBC programming

I understand how to do this on paper in SQL, but am having trouble implementing this in Java (this is the first time I am actually programming JDBC stuff)
For example, say my database consists of:
movie(code, title, publisher)
customer(custno, name)
borrowed(custno, code)
And I want to find the name of customers who borrowed every movie by pubisher ABC
string no_of_ABC_movies = "SELECT COUNT(publisher), publisher FROM movie, WHERE movie.publisher = 'ABC'";
string no_of_cust_ABC_movies = "SELECT COUNT(name), name FROM customer, borrowed, movie, WHERE customer.custno = borrowed.custno AND borrowed.code = movie.code AND movie.publisher = 'ABC'";
String query = "SELECT name" +
" name FROM customer, borrowed, movie" +
" WHERE customer.custno = borrowed.custno AND" +
" borrowed.code = movie.code AND" +
" movie.publisher = 'ABC' AND" + " "
no_of_cust_ABC_movies + " = " + no_of_ABC_movies;
This isn't the exact database I am working with, but query will work and print out the names of people who borrowed movies from ABC without the last line, but says I have an error in SQL syntax with the last line so I guess I don't know how to use one query within another.

It depends on your DBMS, but every SQL variant I've seen requires parens around subqueries.
Try something like:
...
" movie.publisher = 'ABC' AND ("
no_of_cust_ABC_movies + ") = (" + no_of_ABC_movies + ")";

You have problem with double name field without being separated by a comma in your query.
If your code is exactly as listed above, you have compilation error just above the last line-missing + to concatenate strings.
If that's a typo below is my suggestion.
Remove duplicate select (use only one name) or
Separate names by a comma ( I don't see a point of selecting name twice though)
And your last line is wrong.. you can not compare two select queries that way.. Just add the required where clauses.
(You should read database joins first, and then solve your problem)

I like to get my queries working in the query browser or workbench, then copy them over to Java. It keeps it to one new thing at a time...

You're query actually starts with
SELECT name name FROM customer ...
The name column is duplicated - maybe that the problem.

How to write native SQL queries in Hibernate without hardcoding table names and fields?

Sometimes you have to write some of your queries in native SQL rather than hibernate HQL. Is there a nice way to avoid hardcoding table names and fields and get this data from existing mapping?
For example instead of:
String sql = "select user_name from tbl_user where user_id = :id";
something like:
String sql = "select " + Hibernate.getFieldName("user.name") + " from " + Hibernate.getTableName(User.class) + " where " + Hibernate.getFieldName("user.id") + " = :id";

It is possible to get this information as shown below, but I am not sure that I would do this in production code unless I really need the table names to be changeable after the code has been compiled. Otherwise, is it really worth the readability cost to your code to have this?
AbstractEntityPersister metadata =
(AbstractEntityPersister) sessionFactory.getClassMetadata(User.class);
String tableName = metadata.getTableName();
String[] columnNames = metadata.getKeyColumnNames();