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Checking if table exist or not

I am retrieving data from database using jdbc. In my code I am using 3-4 tables to get data. But sometimes if table is not present in database my code gives exception. How to handle this situation. I want my code to continue working for other tables even if one table is not present. Please help.
I have wrote a code like this
sql="select * from table"
now Result set and all.
If table is not present in database it give exception that no such table. I want to handle it. In this code I cannot take tables which are already present in advance . I want to check here itself if table is there or not.
Please do not mark it as a duplicate question. The link you shared doesnot give me required answer as in that question they are executing queries in database not through JDBC code

For Sybase ASE the easiest/quickest method would consist of querying the sysobjects table in the database where you expect the (user-defined) table to reside:
select 1 from sysobjects where name = 'table-name' and type = 'U'
if a record is returned => table exists
if no record is returned => table does not exist
How you use the (above) query is up to you ...
return a 0/1-row result set to your client
assign a value to a #variable
place in a if [not] exists(...) construct
use in a case statement
If you know for a fact that there won't be any other object types (eg, proc, trigger, view, UDF) in the database with the name in question then you could also use the object_id() function, eg:
select object_id('table-name')
if you receive a number => the object exists
if you receive a NULL => the object does not exist
While object_id() will obtain an object's id from the sysobjects table, it does not check for the object type, eg, the (above) query will return a number if there's a stored proc named 'table-name'.
As with the select/sysobjects query, how you use the function call in your code is up to you (eg, result set, populate #variable, if [not] exists() construct, case statement).
So, addressing the additional details provided in the comments ...
Assuming you're submitting a single batch that needs to determine table existence prior to running the desired query(s):
-- if table exists, run query(s); obviously if table does not exist then query(s) is not run
if exists(select 1 from sysobjects where name = 'table-name' and type = 'U')
begin
execute("select * from table-name")
end
execute() is required to keep the optimizer from generating an error that the table does not exist, ie, the query is not parsed/compiled unless the execute() is actually invoked
If your application can be written to use multiple batches, something like the following should also work:
# application specific code; I don't work with java but the gist of the operation would be ...
run-query-in-db("select 1 from sysobjects where name = 'table-name' and type = 'U'")
if-query-returns-a-row
then
run-query-in-db("select * from table-name")
fi

This is the way of checking if the table exists and drop it:
IF EXISTS (
SELECT 1
FROM sysobjects
WHERE name = 'a_table'
AND type = 'U'
)
DROP TABLE a_table
GO
And this is how to check if a table exists and create it.
IF NOT EXISTS (
SELECT 1
FROM sysobjects
WHERE name = 'a_table'
AND type = 'U'
)
EXECUTE("CREATE TABLE a_table (
col1 int not null,
col2 int null
)")
GO
(They are different because in table-drop a temporary table gets created, so if you try to create a new one you will get an exception that it already exists)

Before running the query which has some risk in table not existing, run the following sql query and check if the number of results is >= 1. if it is >= 1 then you are safe to execute the normal query. otherwise, do something to handle this situation.
SELECT count(*)
FROM information_schema.TABLES
WHERE (TABLE_SCHEMA = 'your_db_name') AND (TABLE_NAME = 'name_of_table')
I am no expert in Sybase but take a look at this,
exec sp_tables '%', '%', 'master', "'TABLE'"
Sybase Admin

Invalid character encountered error - selecting rows

I am trying to get rows from table using SQLDeveloper by writing simple query:
SELECT * FROM agreements WHERE agreementkey = 1;
SELECT * FROM agreements WHERE agreementkey = 4;
but getting invalid character encountered error. It's not a problem with query(working using other keys, i.e. agreementkey = 3) but with XMLType column in this table - there is something wrong with data in some rows. Is there a way to select this affected row(I know keys of this affected rows) using queries? Maybe export to file or something? Solution of copying value manually is not acceptable.

Create an empty copy of the table and then run an INSERT into it based on a select from the original table but do it using the DML error logging clause.
This should show you any rows that fail to load and the reason for the failure.
CREATE TABLE test_agreements
AS SELECT * FROM agreements
WHERE ROWNUM <1;
INSERT INTO test_agreements
SELECT *
FROM agreements
LOG ERRORS REJECT LIMIT UNLIMITED
This will create you an error logging table called ERR$TEST_AGREEMENTS which you can query to find the problem rows.

Problem is in WHERE key = 1 cause key is a Reserve Word in Oracle. You should escape it like
SELECT * FROM table WHERE "key" = 1;

KEY is a reserved word so to overcome that you need to use double quotes "".
SELECT * FROM table WHERE "key" = 1;

I think the problem can be solved by putting the argument in quotes:
SELECT * FROM agreements WHERE agreementkey = "1";
I wish I were familiar with XML, but I have run into this with VARCHAR2 columns that are supposed to have all numeric values. Oracle looks at the request
where agreementkey = 1
and tries to convert agreementkey to a number rather than 1 to a varchar2.

If the database contains invalid characters I would try one the following:
Maybe the solution of BriteSponge will work, using an insert statemant with error logging clause.
Use datapump to export the data to a file. I think the log will contain information to identify the invalid columns.
There was a tool called "character set scanner" that checked the characters of the data of a table, here is some documentation: CSSCAN. Or maybe you can use the Database Migration Assistent for Unicode (DMU) mentioned in the same manual.
4- You can write a small PL/SQL program that retrieves the rows row by row and in case of an error catches the exception and notifies you about the row.
DECLARE
invalid_character_detected EXCEPTION;
PRAGMA EXCEPTION_INIT(invalid_character_detected, ???); begin
for SELECT rowid into rec FROM agreements do begin
for
SELECT * into dummy
FROM agreements
where rowid=rec.rowid
do
null;
end loop;
except
WHEN invalid_character_detected THEN
dbms_ouput.put_line(rec.rowid)
end;
end loop;
end;
I did not compile and test the program. ??? is the (negative) error code, e.g. -XXXXX if the error is ORA-XXXXX

SQL Update : WHERE clause error

I don't know whats wrong with this query :
SQL = "UPDATE "+ choosenClass +" SET COUNT = "+count+" WHERE NAME = "+names;
stmt2.executeUpdate( SQL );
Error :
java.sql.SQLSyntaxErrorException: Column 'RAMAYAH' is either not in any table in the FROM list or appears within a join specification and is outside the scope of the join specification or appears in a HAVING clause and is not in the GROUP BY list. If this is a CREATE or ALTER TABLE statement then 'RAMAYAH' is not a column in the target table.
It says Column RAMAYAH is not there but the string "RAMAYAH" itself is taken from the same table. Help Please.

Try to enclose the string into single quotes:
SQL = "UPDATE "+ choosenClass +" SET COUNT = "+count+" WHERE NAME = '"+names+"'";
stmt2.executeUpdate( SQL );

Apart from SQL-Injection vulnerability as mentioned by others in comment; your query has few more mistakes/issues.
You need to quote the filter value in WHERE condition WHERE NAME = "+names should be
WHERE NAME = '"+names+"'". Otherwise, your query will be formed like below and query engine will presume that there exist a column named RAMAYAH in your table in question.
UPDATE tab1 SET COUNT = 10 WHERE NAME = RAMAYAH;
Also, in your UPDATE statement, you have a column names COUNT which is a reserve word. You need to escape it to be in safe side.
If you are using mysql then use backtique
for sql-server use [] square bracket
for postgresql and oracle use "" double-quote

Derby Database : delete records which their timestamp greater than 2 hours

i tried to write a named query in my Entity Java Bean class , and tried also to write the same query as a native query , its job is to delete records which difference between their timestamp column and current time stamp, are greater than 2 hours.
my query :
DELETE FROM APP.WEATHER WHERE timestampdiff(SQL_TSI_HOUR,APP.WEATHER.SINCE,CURRENT_TIMESTAMP) > 2;
but it failed , and this error message appeared to me :
Error code -1, SQL state 42X04: Column 'SQL_TSI_HOUR' is either not in any table in the FROM list or appears within a join specification and is outside the scope of the join specification or appears in a HAVING clause and is not in the GROUP BY list. If this is a CREATE or ALTER TABLE statement then 'SQL_TSI_HOUR' is not a column in the target table.
Line 1, column 1

How to invertly select columns in sql?

If I have a SQL table with columns:
NR_A, NR_B, NR_C, NR_D, R_A, R_B, R_C
and on runtime, I add columns following the column's sequence such that the next column above would be R_D followed by R_E.
My problem is I need to reset the values of columns that starts with R_ (labeled that way to indicate that it is resettable) back to 0 each time I re-run my script . NR_ columns btw are fixed, so it is simpler to just say something like:
UPDATE table set col = 0 where column name starts with 'NR_'
I know that is not a valid SQL but I think its the best way to state my problem.
Any thoughts?
EDIT: btw, I use postgres (if that would help) and java.

SQL doesn't support dynamically named columns or tables--your options are:
statically define column references
use dynamic SQL to generate & execute the query/queries
Java PreparedStatements do not insulate you from this--they have the same issue, just in Java.

Are you sure you have to add columns during normal operations? Dynamic datamodels are most of the time a realy bad idea. You will see locking and performance problems.
If you need a dynamic datamodel, take a look at key-value storage. PostgreSQL also has the extension hstore, check the contrib.

If you don't have many columns and you don't expect the schema to change, just list them explicitly.
UPDATE table SET NR_A=0;
UPDATE table SET NR_B=0;
UPDATE table SET NR_C=0;
UPDATE table SET NR_D=0;
Otherwise, a simple php script could dynamically build and execute your query:
<?php
$db = pg_connect("host=localhost port=5432 user=postgres password=mypass dbname=mydb");
if(!$db) die("Failed to connect");
$reset_cols = ["A","B","C","D"];
foreach ($col in $reset_cols) {
$sql = "UPDATE my_table SET NR_" . $col . "=0";
pg_query($db,$sql);
}
?>
You could also lookup table's columns in Postgresql by querying the information schema columns tables, but you'll likely need to write a plpgsql function to loop over the query results (one row per table column starting with "NR_").

if you rather using sql query script, you should try to get the all column based on given tablename.
maybe you could try this query to get all column based on given tablename to use in your query.
SELECT attname FROM
pg_attribute, pg_type
WHERE typname = 'tablename' --your table name
AND attrelid = typrelid
AND attname NOT IN ('cmin', 'cmax', 'ctid', 'oid', 'tableoid', 'xmin', 'xmax')
--note that this attname is sys column
the query would return all column with given tablename except system column