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I'd like to convert this SQL Statement into either Criteria or Projections. I'm sorry I don't know which one to use since I'm new to Hibernate. I've done some research, and it looks like both are needed to achieve what I wanted.
Before I always use SQL Statements in my programs but when I saw a sample code which allows you to create queries without writing SQL Statements by using Criteria, I wanted to use it since it gives me cleaner code.
Thanks in advance.
String query = "SELECT MAX(employeeNo) "
+ "FROM Profile "
+ "WHERE SUBSTRING(employeeNo,1,2)='" + yy + "'";
empNo = session.createQuery(query).list().get(0).toString();
This query gets the highest employee number in the database for a given year yy.
Sample result would be 14-229
This is assuming that you've already defined a mapping for Profile entity.
Session session = sessionFactory.getCurrentSession();
String empNo = (String) session
.createCriteria(Profile.class)
.add(Restrictions.sqlRestriction("substring(employeeNo,1,2) = ?", yy, StringType.INSTANCE)
.setProjection(Projections.max("employeeNo"))
.uniqueResult();
Now, this I don't know if I'd agree with your statement that this is cleaner. The nice thing about using Criteria is the allow you to build a criteria dynamically, but it's very verbose, and it isn't clear exactly what you are trying to do at first glance. Plus, because of the substring function, forces you to mix in a sqlRestriction. Instead, you could use HQL:
Session session = sessionFactory.getCurrentSession();
String empNo = (String) session.createQuery("SELECT MAX(employeeNo) FROM Profile WHERE substring(employeeNo,1,2) = :year")
.setString("year", yy)
.uniqueResult();
Personally, I think the HQL or even SQL approach is a little cleaner, and expresses intent better.
This may be really basic, but I can't think of how to write a SQL query that would find strings that have the same characters but different capitalization.
The context I'm working on is a Rails 3.2 app. I have a simple Tag model with a Name attribute. I've inherited data for this model that did not store values case-insensitively, so some users input things like "Tree" while others input "tree" and now we have two tags that really should be one.
So, I'd like to do a query to find all these pairs so that I can go about merging them.
The only thing I can think of so far is to write a rake task that loops through them all and checks for matching values... something like:
pairs = []
Tag.all.each do |t|
other = Tag.where( 'name LIKE ?', t.name )
pairs << [t, other] if other
end
However, I'm not sure the above would work, or that it makes sense performance-wise. Is there a better way to write a SQL query that would find these matching pairs?
There is a question similar to this here
What you can do is take that answer a create a method in your model to do a case insensitive search. From what i've experience however is that ActiveRecord already does case insensitive search but just in case:
def self.insensitive_find_by_tag_name(name)
Tag.where("lower(name) = ? ", name.downcase)
end
and then to remove duplicate entries, you can do something like this
Tag.transaction! do
tags = Tag.insensitive_find_by_tag_name(name)
tags.last(tags.length() - 1).each do |tag|
tag.destroy
end
end
Call transaction just in case anything fails so the database will rollback. Grab all tags with the same name, then delete any extra entries. If you want the remaining tag entry to be lower case then you can do
tag = tags.first
tag.name = tag.name.downcase
tag.save!
I'm not super good at SQL, but I researched this a bit and found out that using the COLLATE clause can be used to make string operations case sensitive in SQL. (typically select distinct operations are case insensitive.)
so maybe you could try:
select distinct (name) COLLATE sql_latin1_general_cp1_cs_as
FROM (
... blah blah blah
Here is some documentation on collate:
http://dev.mysql.com/doc/refman/5.0/en/charset-collate.html
(assuming you're using mysql I guess)
Alternatively you could also reconfigure your database to be case sensitive via collate also. Then your current query might work unaltered
(assuming you have administrative permissions and ability to reconfigure)
You should use upper() or lower() functions to convert the names all to lower or upper case.
SELECT DISTINCT upper(name)
Or:
SELECT DISTINCT lower(name)
Source: http://www.postgresql.org/docs/9.1/static/functions-string.html
Another option (better for maintainability of code) is to use the CITEXT type, but to do this you have to modify your table structure: http://www.postgresql.org/docs/9.1/static/citext.html
I saw a query run in a log file on an application. and it contained a query like:
SELECT ID FROM CUST_ATTR49 WHERE 1=0
what is the use of such a query that is bound to return nothing?
A query like this can be used to ping the database. The clause:
WHERE 1=0
Ensures that non data is sent back, so no CPU charge, no Network traffic or other resource consumption.
A query like that can test for:
server availability
CUST_ATTR49 table existence
ID column existence
Keeping a connection alive
Cause a trigger to fire without changing any rows (with the where clause, but not in a select query)
manage many OR conditions in dynamic queries (e.g WHERE 1=0 OR <condition>)
This may be also used to extract the table schema from a table without extracting any data inside that table. As Andrea Colleoni said those will be the other benefits of using this.
A usecase I can think of: you have a filter form where you don't want to have any search results. If you specify some filter, they get added to the where clause.
Or it's usually used if you have to create a sql query by hand. E.g. you don't want to check whether the where clause is empty or not..and you can just add stuff like this:
where := "WHERE 0=1"
if X then where := where + " OR ... "
if Y then where := where + " OR ... "
(if you connect the clauses with OR you need 0=1, if you have AND you have 1=1)
As an answer - but also as further clarification to what #AndreaColleoni already mentioned:
manage many OR conditions in dynamic queries (e.g WHERE 1=0 OR <condition>)
Purpose as an on/off switch
I am using this as a switch (on/off) statement for portions of my Query.
If I were to use
WHERE 1=1
AND (0=? OR first_name = ?)
AND (0=? OR last_name = ?)
Then I can use the first bind variable (?) to turn on or off the first_name search criterium. , and the third bind variable (?) to turn on or off the last_name criterium.
I have also added a literal 1=1 just for esthetics so the text of the query aligns nicely.
For just those two criteria, it does not appear that helpful, as one might thing it is just easier to do the same by dynamically building your WHERE condition by either putting only first_name or last_name, or both, or none. So your code will have to dynamically build 4 versions of the same query. Imagine what would happen if you have 10 different criteria to consider, then how many combinations of the same query will you have to manage then?
Compile Time Optimization
I also might add that adding in the 0=? as a bind variable switch will not work very well if all your criteria are indexed. The run time optimizer that will select appropriate indexes and execution plans, might just not see the cost benefit of using the index in those slightly more complex predicates. Hence I usally advice, to inject the 0 / 1 explicitly into your query (string concatenating it in in your sql, or doing some search/replace). Doing so will give the compiler the chance to optimize out redundant statements, and give the Runtime Executer a much simpler query to look at.
(0=1 OR cond = ?) --> (cond = ?)
(0=0 OR cond = ?) --> Always True (ignore predicate)
In the second statement above the compiler knows that it never has to even consider the second part of the condition (cond = ?), and it will simply remove the entire predicate. If it were a bind variable, the compiler could never have accomplished this.
Because you are simply, and forcedly, injecting a 0/1, there is zero chance of SQL injections.
In my SQL's, as one approach, I typically place my sql injection points as ${literal_name}, and I then simply search/replace using a regex any ${...} occurrence with the appropriate literal, before I even let the compiler have a stab at it. This basically leads to a query stored as follows:
WHERE 1=1
AND (0=${cond1_enabled} OR cond1 = ?)
AND (0=${cond2_enabled} OR cond2 = ?)
Looks good, easily understood, the compiler handles it well, and the Runtime Cost Based Optimizer understands it better and will have a higher likelihood of selecting the right index.
I take special care in what I inject. Prime way for passing variables is and remains bind variables for all the obvious reasons.
This is very good in metadata fetching and makes thing generic.
Many DBs have optimizer so they will not actually execute it but its still a valid SQL statement and should execute on all DBs.
This will not fetch any result, but you know column names are valid, data types etc. If it does not execute you know something is wrong with DB(not up etc.)
So many generic programs execute this dummy statement for testing and fetching metadata.
Some systems use scripts and can dynamically set selected records to be hidden from a full list; so a false condition needs to be passed to the SQL. For example, three records out of 500 may be marked as Privacy for medical reasons and should not be visible to everyone. A dynamic query will control the 500 records are visible to those in HR, while 497 are visible to managers. A value would be passed to the SQL clause that is conditionally set, i.e. ' WHERE 1=1 ' or ' WHERE 1=0 ', depending who is logged into the system.
quoted from Greg
If the list of conditions is not known at compile time and is instead
built at run time, you don't have to worry about whether you have one
or more than one condition. You can generate them all like:
and
and concatenate them all together. With the 1=1 at the start, the
initial and has something to associate with.
I've never seen this used for any kind of injection protection, as you
say it doesn't seem like it would help much. I have seen it used as an
implementation convenience. The SQL query engine will end up ignoring
the 1=1 so it should have no performance impact.
Why would someone use WHERE 1=1 AND <conditions> in a SQL clause?
If the user intends to only append records, then the fastest method is open the recordset without returning any existing records.
It can be useful when only table metadata is desired in an application. For example, if you are writing a JDBC application and want to get the column display size of columns in the table.
Pasting a code snippet here
String query = "SELECT * from <Table_name> where 1=0";
PreparedStatement stmt = connection.prepareStatement(query);
ResultSet rs = stmt.executeQuery();
ResultSetMetaData rsMD = rs.getMetaData();
int columnCount = rsMD.getColumnCount();
for(int i=0;i<columnCount;i++) {
System.out.println("Column display size is: " + rsMD.getColumnDisplaySize(i+1));
}
Here having a query like "select * from table" can cause performance issues if you are dealing with huge data because it will try to fetch all the records from the table. Instead if you provide a query like "select * from table where 1=0" then it will fetch only table metadata and not the records so it will be efficient.
Per user milso in another thread, another purpose for "WHERE 1=0":
CREATE TABLE New_table_name as select * FROM Old_table_name WHERE 1 =
2;
this will create a new table with same schema as old table. (Very
handy if you want to load some data for compares)
An example of using a where condition of 1=0 is found in the Northwind 2007 database. On the main page the New Customer Order and New Purchase Order command buttons use embedded macros with the Where Condition set to 1=0. This opens the form with a filter that forces the sub-form to display only records related to the parent form. This can be verified by opening either of those forms from the tree without using the macro. When opened this way all records are displayed by the sub-form.
In ActiveRecord ORM, part of RubyOnRails:
Post.where(category_id: []).to_sql
# => SELECT * FROM posts WHERE 1=0
This is presumably because the following is invalid (at least in Postgres):
select id FROM bookings WHERE office_id IN ()
It seems like, that someone is trying to hack your database. It looks like someone tried mysql injection. You can read more about it here: Mysql Injection
Simple, a SQL injection attack in as few characters as possible. Note, I'm not trying to prevent SQL injection attacks by limiting inputs to a certain size, but rather am genuinely curious how many characters is needed to execute even the simplest attack.
For posterity sake, let's say the smallest table name is 4 characters, e.g., "user". Please factor that in.
1 Character is the smallest unit that you have control over. The question depends heavily on what you're doing. For instance, if you're dealing with an interface to delete your profile from a site, and you send '%' instead of your name:
"Delete from Users where name like '"+username+"'"
then setting your username to % will delete all the users.
When injecting into a string literal:
';drop database;--
Suppose the query was generated like this
"Select * from user where userid = " + myVar
1; delete from user;
how about this one:
' OR 1=1 --
Ok I need to build a query based on some user input to filter the results.
The query basically goes something like this:
SELECT * FROM my_table ORDER BY ordering_fld;
There are four text boxes in which users can choose to filter the data, meaning I'd have to dynamically build a "WHERE" clause into it for the first filter used and then "AND" clauses for each subsequent filter entered.
Because I'm too lazy to do this, I've just made every filter an "AND" clause and put a "WHERE 1" clause in the query by default.
So now I have:
SELECT * FROM my_table WHERE 1 {AND filters} ORDER BY ordering_fld;
So my question is, have I done something that will adversely affect the performance of my query or buggered anything else up in any way I should be remotely worried about?
MySQL will optimize your 1 away.
I just ran this query on my test database:
EXPLAIN EXTENDED
SELECT *
FROM t_source
WHERE 1 AND id < 100
and it gave me the following description:
select `test`.`t_source`.`id` AS `id`,`test`.`t_source`.`value` AS `value`,`test`.`t_source`.`val` AS `val`,`test`.`t_source`.`nid` AS `nid` from `test`.`t_source` where (`test`.`t_source`.`id` < 100)
As you can see, no 1 at all.
The documentation on WHERE clause optimization in MySQL mentions this:
Constant folding:
(a<b AND b=c) AND a=5
-> b>5 AND b=c AND a=5
Constant condition removal (needed because of constant folding):
(B>=5 AND B=5) OR (B=6 AND 5=5) OR (B=7 AND 5=6)
-> B=5 OR B=6
Note 5 = 5 and 5 = 6 parts in the example above.
You can EXPLAIN your query:
http://dev.mysql.com/doc/refman/5.0/en/explain.html
and see if it does anything differently, which I doubt. I would use 1=1, just so it is more clear.
You might want to add LIMIT 1000 or something, when no parameters are used and the table gets large, will you really want to return everything?
WHERE 1 is a constant, deterministic expression which will be "optimized out" by any decent DB engine.
If there is a good way in your chosen language to avoid building SQL yourself, use that instead. I like Python and Django, and the Django ORM makes it very easy to filter results based on user input.
If you are committed to building the SQL yourself, be sure to sanitize user inputs against SQL injection, and try to encapsulate SQL building in a separate module from your filter logic.
Also, query performance should not be your concern until it becomes a problem, which it probably won't until you have thousands or millions of rows. And when it does come time to optimize, adding a few indexes on columns used for WHERE and JOIN goes a long way.
TO improve performance, use column indexes on fields listen in "WHERE"
Standard SQL Injection Disclaimers here...
One thing you could do, to avoid SQL injection since you know it's only four parameters is use a stored procedure where you pass values for the fields or NULL. I am not sure of mySQL stored proc syntax, but the query would boil down to
SELECT *
FROM my_table
WHERE Field1 = ISNULL(#Field1, Field1)
AND Field2 = ISNULL(#Field2, Field2)
...
ORDRE BY ordering_fld
We've been doing something similiar not too long ago and there're a few things that we observed:
Setting up the indexes on the columns we were (possibly) filtering, improved performance
The WHERE 1 part can be left out completely if the filters're not used. (not sure if it applies to your case) Doesn't make a difference, but 'feels' right.
SQL injection shouldn't be forgotten
Also, if you 'only' have 4 filters, you could build up a stored procedure and pass in null values and check for them. (just like n8wrl suggested in the meantime)
That will work - some considerations:
About dynamically built SQL in general, some databases (Oracle at least) will cache execution plans for queries, so if you end up running the same query many times it won't have to completely start over from scratch. If you use dynamically built SQL, you are creating a different query each time so to the database it will look like 100 different queries instead of 100 runs of the same query.
You'd probably just need to measure the performance to find out if it works well enough for you.
Do you need all the columns? Explicitly specifying them is probably better than using * anyways because:
You can visually see what columns are being returned
If you add or remove columns to the table later, they won't change your interface
Not bad, i didn't know this snippet to get rid of the 'is it the first filter 3' question.
Tho you should be ashamed of your code ( ^^ ), it doesn't do anything to performance as any DB Engine will optimize it.
The only reason I've used WHERE 1 = 1 is for dynamic SQL; it's a hack to make appending WHERE clauses easier by using AND .... It is not something I would include in my SQL otherwise - it does nothing to affect the query overall because it always evaluates as being true and does not hit the table(s) involved so there aren't any index lookups or table scans based on it.
I can't speak to how MySQL handles optional criteria, but I know that using the following:
WHERE (#param IS NULL OR t.column = #param)
...is the typical way of handling optional parameters. COALESCE and ISNULL are not ideal because the query is still utilizing indexes (or worse, table scans) based on a sentinel value. The example I provided won't hit the table unless a value has been provided.
That said, my experience with Oracle (9i, 10g) has shown that it doesn't handle [ WHERE (#param IS NULL OR t.column = #param) ] very well. I saw a huge performance gain by converting the SQL to be dynamic, and used CONTEXT variables to determine what to add. My impression of SQL Server 2005 is that these are handled better.
I have usually done something like this:
for(int i=0; i<numConditions; i++) {
sql += (i == 0 ? "WHERE " : "AND ");
sql += dbFieldNames[i] + " = " + safeVariableValues[i];
}
Makes the generated query a little cleaner.
One alternative i sometimes use is to build the where clause an an array and then join them together:
my #wherefields;
foreach $c (#conditionfields) {
push #wherefields, "$c = ?",
}
my $sql = "select * from table";
if(#wherefields) { $sql.=" WHERE " . join (" AND ", #wherefields); }
The above is written in perl, but most languages have some kind of join funciton.