I have some tables in a database. They have some particular pattern. For example, consider I have table employee, then some other table with same pattern like:
table 1:employee
table 2:employee_X
table 3:employee_Y
I want to check if these tables contain data or not and if they do then I have to call some method for each table. I am using following code to retrieve.
DatabaseMetaData meta = con.getMetaData();
ResultSet res = meta.getTables(null, null, "My_Table_Name", new String[] {"TABLE"});
while (res.next()) {
if(rs.getStrin(3).equals(employee)){
//my code to write data of this table to a file
}
if(rs.getString(3).equals(employee_X)){
//my code to write data to the same file
}
if(rs.getString(3).equals(employee_Y)){
//code to write data to the same file from this table
}
}
The code is working fine, but how I can retrieve data from all these tables at once instead of using three checks. If any of these table contains data I want to write it to my file. How I can perform this operation in less lines of code and efficiently?
It would be great if anyone can suggest way to check each of these table either contain data or not in a single statement and then I can call my code to write data to file.
You can use UNION statement in your complex query. Please, check example:
SELECT id, name FROM employee WHERE id = ?
UNION
SELECT id, name FROM employee_x WHERE id = ?
UNION
...
Also you can use UNION ALL statement instead of UNION. The main difference that UNION returns unique result set without duplicates, UNION ALL allows duplicates. Please, check this link https://www.w3schools.com/sql/sql_union.asp for detailed explanation about union statement.
If you need create UNION query with custom filtered tables, please check example:
Set<String> requiredTables = new HashSet<>();
// fill set with required tables for result query
requiredTables.add("employee");
ResultSet res = meta.getTables(null, null, "My_Table_Name",
new String[] {"TABLE"});
List<String> existentTables = new LinkedList<>();
while(res.next()) {
if (requiredTables.contains(res.getString(3)) {
existentTables.add(res.getString(3));
}
}
String query = existentTables.stream().map(table -> String.format("SELECT * FROM %s", table)).collect(Collectors.joinning(" UNION "));
Related
I am trying to generate sql query based on user input. There are 4 search fields on the UI:
FIRST_NAME, LAST_NAME, SUBJECT, MARKS
Based on user input I am planning to generate SQL query. Input can be of any combination.
eg: select * from TABLE where FIRST_NAME="some_value";
This query needs to be generated when FIRST_NAME is given and other fields are null
select * from TABLE where FIRST_NAME="some_value" and LAST_NAME="some_value";
This query needs to be generated when FIRST_NAME and LAST_NAME are given and other fields are null
Since there are 4 input fields, number of possible queries that can be generated are 24 (factorial of 4).
One idea is to write if condition for all 24 cases.
Java pseudo code:
String QUERY = "select * from TABLE where ";
if (FIRST_NAME!=null) {
QUERY = QUERY + "FIRST_NAME='use_input_value';"
}
if (LAST_NAME!=null) {
QUERY = QUERY + "LAST_NAME='use_input_value';"
}
if (SUBJECT!=null) {
QUERY = QUERY + "SUBJECT='use_input_value';"
}
if (MARKS!=null) {
QUERY = QUERY + "MARKS='use_input_value';"
}
I am not able to figure out how to generate SQL queries with AND coditions for multiple Input values.
I have been through concepts on dynamically generate sql query but couldn't process further.
Can someone help me on this.
FYI: I have been through How to dynamically generate SQL query based on user's selections?, still not able to generate query string based on user input.
Let's think about what would happen if you just ran the code you wrote and both FIRST_NAME and LAST_NAME are provided. You'll wind up with this:
select * from TABLE where FIRST_NAME='use_input_value';LAST_NAME='use_input_value';
There are two problems here:
The query is syntactically incorrect.
It contains the literals 'use_input_value' instead of the values you want.
To fix the first problem, let's first add and to the start of each expression, and remove the semicolons, something like this:
String QUERY = "select * from TABLE where";
if (FIRST_NAME!=null) {
QUERY = QUERY + " and FIRST_NAME='use_input_value'";
}
Notice the space before the and. We can also remove the space after where.
Now the query with both FIRST_NAME and LAST_NAME will look like this:
select * from TABLE where and FIRST_NAME='use_input_value' and LAST_NAME='use_input_value'
Better but now there's an extra and. We can fix that by adding a dummy always-true condition at the start of the query:
String QUERY = "select * from TABLE where 1=1";
Then we append a semicolon after all the conditions have been evaluated, and we have a valid query:
select * from TABLE where 1=1 and FIRST_NAME='use_input_value' and LAST_NAME='use_input_value';
(It may not be necessary to append the semicolon. Most databases don't require semicolons at the end of a single query like this.)
On to the string literals. You should add a placeholder instead, and simultaneously add the value you want to use to a List.
String QUERY = "select * from TABLE where";
List<String> args = new ArrayList<>();
if (FIRST_NAME!=null) {
QUERY = QUERY + " and FIRST_NAME=?";
args.add(FIRST_NAME);
}
After you've handled all the conditions you'll have a string with N '?' placeholders and a List with N values. At that point just prepare a query from the SQL string and add the placeholders.
PreparedStatement statement = conn.prepareStatement(QUERY);
for (int i = 0; i < args.size(); i++) {
statement.setString(i + 1, args[i]);
}
For some reason columns and parameters are indexed starting at 1 in the JDBC API, so we have to add 1 to i to produce the parameter index.
Then execute the PreparedStatement.
I am using jdbc to retrieve data from database. I have four different queries for that, however, the result of first query is used to get the data of second and fourth query. But, the resultset, gets updated as i run other queries. So is there any way that i can keep the resultset and add new results in it.
Here is my code:
class GetData{
String toDate;
String fromDate;
GetData(String d1,String d2) throws ClassNotFoundException, SQLException, ParseException, TransformerException, ParserConfigurationException {
toDate=d1;
fromDate=d2;
Connection connection= null;
ResultSet resultset= null;
String customerquery="SELECT o.ordernumber,o.orderdate,o.customernumber,c.customername,c.addressLine1,c.postalCode,c.city,c.country from orders o join customers c on o.customernumber=c.customernumber where orderdate between ? and ?";
String orderdetailquery="SELECT orderNumber,productCode,quantityOrdered,priceEach,orderLineNumber,(quantityOrdered * priceEach) as total FROM orderdetails where ordernumber=?";
String productsquery="SELECT productName,productLine,productVendor FROM products where productcode=?";
String employeequery="SELECT c.salesRepEmployeeNumber,e.firstname,e.lastname,o.officecode,o.city from customers c join employees e on c.salesRepEmployeeNumber = e.employeeNumber join offices o on e.officecode=o.officecode where c.customernumber=?";
Class.forName("com.mysql.cj.jdbc.Driver");
connection=DriverManager.getConnection("jdbc:mysql://localhost:3306/classicmodels","root","root");
if(d1!=null || d2!=null) {
PreparedStatement p1= connection.prepareStatement(customerquery);
p1.setString(1, toDate);
p1.setString(2,fromDate);
resultset= p1.executeQuery();
PreparedStatement p2= connection.prepareStatement(orderdetailquery);
while(resultset.next()) {
p2.setString(1, resultset.getString("orderNumber"));
}
resultset=p2.executeQuery();
PreparedStatement p3= connection.prepareStatement(productsquery);
while (resultset.next()) {
p3.setString(1, resultset.getString("productcode"));
}
resultset=p3.executeQuery();
PreparedStatement p4=connection.prepareStatement(employeequery);
while(resultset.next()) {
p4.setString(1, resultset.getString("customernumber"));
}
resultset=p4.executeQuery();
resultset.close();
connection.close();
}
}
I am trying to use the result of first query that contains the column customernumber to fetch the data. While, executing the code a error comes that customernumber column not found. So, how can I use the result of first query in other queries. Also, i am trying to get all the result of queries in one resultset as I am trying to create a xml out of it by using DOM.
You can put everything in a single query. using joins.
select o.ordernumber,o.orderdate
, o.customernumber,c.customername
, c.addressLine1,c.postalCode,c.city,c.country
, t1.productCode, t1.quantityOrdered, t1.priceEach, t1.orderLineNumber,(t1.quantityOrdered * t1. priceEach) as total
, t2.productName, t2.productLine, t2.productVendor
, c.salesRepEmployeeNumber,t4.firstname,t4.lastname,t3.officecode,t3.city
from orders o
join customers c on o.customernumber=c.customernumber
left join orderdetails t1 on t1.orderNumber = o.orderNumber
left join products t2 on t2.productCode = t1.productCode
left join offices t3 on t3.offiecode = c.customernumber
left join employees t4 on t3.officecode = t4.officecode
ResultSet is getting changed because you are using the same reference (object) every time you are executing the sql statement, hence it is overriding the old result, so if you want to deal with previously returned ResultSet, you can create new ResultSet instance to use, and also you can create a Bean class to set the elements and make it List type to keep on adding the results as per your logic.
Create a POJO to store required properties retrieved from different queries.
The resultset is getting updated because the same reference variable is being used to assign the resultset of subsequent queries.
If you want to use the same variable, you can follow like this -
create a POJO (which can be used to create a xml out of it by using DOM).
get result of first query in resultset.
populate relevant properties of POJO from this resultset.
re-use the resultset to store result of next queries.
When you need to use previous results, get them from the POJO to be used as parameters in subsequent queries.
I have a query using various joins, and I just need the list of columns which are returned by this query. I done it in java, by asking only one row with rownum=1 and getting column name for value.The problem is if there is no data returned by that query.
For ex.
select * from something
and if there is any data returning by this query then it will return col1,col2,col3.
But if there is no data returned by this query, then it will throw error.
What I need is
Is there any way that I can run
desc (select * from something)
or similar to get list of columns returned by query.
It can be in sql or JAVA. Both methods are acceptable.
In my application, user passes the query, and I can add wrapper to the query but I cant modify it totally.
The flow of application is
query from user -> execute by java and get one row -> return list of columns in the result set.
you can use ResultSetMetaData of resultset
for example :
ResultSet rs = stmt.executeQuery("SELECT a, b, c FROM TABLE2");
ResultSetMetaData rsmd = rs.getMetaData();
int countOfColumns = rsmd.getColumnCount();
for(int i = 1; i <= countOfColumns ; i++ )
System.out.println(rsmd.getColumnName(i));
you could maybe convert your query to a view, you can then see the columns in the view by querying user_tab_columns
select * from user_tab_columns
The Oracle equivalent for information_schema.COLUMNS is USER_TAB_COLS for tables owned by the current user, ALL_TAB_COLS or DBA_TAB_COLS for tables owned by all users.
Tablespace is not equivalent to a schema, neither do you have to provide the tablespace name.
Providing the schema/username would be of use if you want to query ALL_TAB_COLS or DBA_TAB_COLS for columns OF tables owned by a specific user. in your case, I'd imagine the query would look something like:
String sqlStr= "
SELECT column_name
FROM all_tab_cols
WHERE table_name = 'users'
AND owner = ' || +_db+ || '
AND column_name NOT IN ( 'password', 'version', 'id' )
"
Note that with this approach, you risk SQL injection.
I want to fetch data from MySQL without creating object from class
Normally I do something like
public ArrayList getInventoryByItemId(String ItemId) throws SQLException {
ArrayList list = new ArrayList<Inventory>();
String sql = "SELECT iid, i.uid, item_data, item_id, i.ctime, username, gender FROM Inventory i JOIN user u ON i.uid = u.uid WHERE item_id = '"+ItemId+"'";
Statement stmt = con.createStatement();
ResultSet rset = stmt.executeQuery(sql);
while (rset.next()) {
a = new Inventory(rset.getInt(1), rset.getInt(2), rset.getString(3), rset.getString(4), rset.getTimestamp(6), rset.getString(7), rset.getString(8));
list.add(a);
}
return list;
}
the problem is because Inventory object does not have user data from the joined user table, I cannot create new Inventory.
I just want to automatically make an object where it has all the data attributes, that I can access using the column name.
Thank You
If I got you problem,
You can create new map(HashMap I reccomend) and put values using column name or index as key.
So, your list will be list of maps.
while (rset.next()) {
a = new HashMap<Integer,Object>();
a.put(1,rset.getInt(1));
..........
list.add(a);
}
Or, If you know exact number of columns, you can user array instead of Map (it will be faster)
Based on what you are saying, I take it that your query is not returning data because the inventory does not have the user data that it is being joined with. You need to modify your query to use a left outer join.
String sql = "SELECT iid, i.uid, item_data, item_id, i.ctime, username, gender FROM Inventory i LEFT JOIN user u ON i.uid = u.uid WHERE item_id = '"+ItemId+"'"
This will allow your query to return Inventory data even if the corresponding User data does not exist.
i have two tables "Table1" with columns user_name,Password and course ID and another table "course" with columns course_id,course_name.I have used the following code to display the course ID from Table1 according to the user_name received from the login page.using ResultSet rs1.now i want to retrieve the course_name from the table "course" according to the course ID receieve from "Table1".for that in the second query pstmt2.setString(1, ); what parameter i should use to get the course_id value from the previous query
HttpSession sess=request.getSession();
String a=(String)sess.getAttribute("user");
String b=(String)sess.getAttribute("pass");
try {
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
Connection con=DriverManager.getConnection("jdbc:odbc:ggg");
Statement st = con.createStatement();
String query="select * from Table1 where user_name=?";
PreparedStatement pstmt=con.prepareStatement(query);
pstmt.setString(1,a);
ResultSet rs1=pstmt.executeQuery();
while(rs1.next())
out.println("<h3>COURSE ID: "+rs1.getString("course ID")+"<h3>");
String query2="SELECT * from course where course_id=?";
PreparedStatement pstmt2=con.prepareStatement(query2);
pstmt2.setString(1,);
ResultSet rs2=pstmt2.executeQuery();
while(rs2.next())
{
out.println("<h3>course name: "+rs2.getString("course_name")+"<h3>");
}
why do you go for two turns of database hit, even though you created one time connection object.
modify the query as below
SELECT * from course where course_id = (select course_id from Table1 where user_name=?);
from this query you noneed to give input of courseid also.
No need to hit database twice to get the results that you need. use the query
Select table1.course_id, course.course_name from table1, course where table1.course_id=course_id and table1.user_name=?
This should set the course_id parameter:
pstmt2.setString(1,rs1.getString("course_id"));
Or, as I see the "course_id" column may have a different name in "Table1":
pstmt2.setString(1,rs1.getString("course ID"));
As the other post mentioned there's no need to go to another set of query. Try this example query:
SELECT course.course_id, course.course_name
FROM table1 t1
INNER JOIN course c
ON t1.course_id = c.course_id
WHERE t1.user_name = ?;
Now if you insist your coding the parameter o your pstmt2.setString(1,); is:
pstmt2.setString(1,rs1.getString("course_id")); //or course ID defending on your column name