may i know how do you check if a database table already exist in the database already or not with just using the sql query method.
Try this
IF Exist(Select 1 from INFORMATION_SCHEMA. COLUMNS where TABLE_NAME like '%tableName%')
Related
How do I drop a few tables (e.g. 1 - 3) using the output of a SELECT statement for the table names? This is probably standard SQL, but specifically I'm using Apache Impala SQL accessed via Apache Zeppelin.
So I have a table called tables_to_drop with a single column called "table_name". This will have one to a few entries in it, each with the name of another temporary table that was generated as the result of other processes. As part of my cleanup I need to drop these temporary tables whose names are listed in the "tables_to_drop" table.
Conceptually I was thinking of an SQL command like:
DROP TABLE (SELECT table_name FROM tables_to_drop);
or:
WITH subquery1 AS (SELECT table_name FROM tables_to_drop) DROP TABLE * FROM subquery1;
Neither of these work (syntax errors). Any ideas please?
even in standard sql this is not possible to do it the way you showed.
in standard sql usually you can use dynamic sql which impala doesn't support.
however you can write an impala script and run it in impala shell but it's going to be complicated for such task, I would prepare the drop statement using select and run it manually if this is one-time thing:
select concat('DROP TABLE IF EXISTS ',table_name) dropstatements
from tables_to_drop
I'm looking for an alternative to the IF-ELSE-Statement as known in MS-SQL for SQLite.
Try searching on stackoverflow first!
I did. I found something like that:
SQLite 'IF NOT EXISTS' syntax error
SQLite if statement in WHERE clause
and so on...
The problem in that cases is that they perform SELECT/INSERT/UPDATE/DELETE.
I want to alter the schema of an existing database.
Creating a new table is trivial because there is CREATE TABLE IF NOT EXISTS "foo" (....);.
But how about adding columns to an existing table?
I want to write a script like:
IF NOT EXISTS (SELECT * FROM pragma_table_info("<table_name>") WHERE name == "<column_name>")
BEGIN
ALTER TABLE "<table_name>" ADD "<column_name>" TEXT;
END
This sounds like ALTER TABLE ADD COLUMN IF NOT EXISTS in SQLite. But this post is out of the year 2010 and I would expect that something has changed in the last 9 years.
I need to do some statements like that using arbitrary queries and statements.
Is there any way to do that in pure SQL or do I have to handle that in application code?
How can I delete the contents of a table only if it exists? Preferably the sql statement should be standard and not oriented to any db.
Please notice that I do not want to drop a table if it exists, i.e.
DROP TABLE IF EXISTS foo
PS: I have already checked truncate and delete but they don't fit the requirement if the table exists.
As far as i know, there is no standard for "if exists". Some databases support it, others do not, and will give you a syntax exception.
Edit
INFORMATION_SCHEMA does not usually change between different versions and is common in most databases, and to my best knowledge this is the most proper way to check whether a table exists in SQL:
IF (EXISTS (SELECT *
FROM INFORMATION_SCHEMA.TABLES
WHERE TABLE_NAME = tableName ))
BEGIN
DELETE FROM tableName
END
I'm using Oracle DB and I would like to write a SQL query that I could then call with JDBC. I'm not very familiar with SQL so if someone can help me, that could be great ! Here is the problem. I have a table MY_TABLE wich contains a list of another tables, and I would like to keep only the nonempty tables and those that their names start by a particular string.
The query I wrote is the following :
select TABLE_NAME
from MY_TABLE
where TABLE_NAME like '%myString%'
and (select count(*) from TABLE_NAME where rownum=1)<>0
order by TABLE_NAME;`
The problem comes from the second SELECT, but I don't know how can I do to use the TABLE_NAME value.
Does someone have an idea ?
Thanks.
[Added from comments]
Actually, I need to test the V$ views contained in the ALL_CATALOG table. But if I can find another table where all these views are contained too and with a NUM_ROWS column too, it would be perfect !
Standard versions of SQL do not allow you to replace 'structural elements' of the query, such as table name or column name, with variable values or place-holders.
There are a few ways to approach this.
Generate a separate SQL statement for each table name listed in MY_TABLE, and execute each in turn. Brute force, but effective.
Interrogate the system catalog directly.
Investigate whether there are JDBC metadata operations that allow you to find out about the number of rows in a table without being tied to the system catalog of the specific DBMS you are using.
Can you use oracle view USER_TABLES? then query will be much easier
select TABLE_NAME
from USER_TABLES
where TABLE_NAME like '%myString%'
and Num_ROWS > 0
order by TABLE_NAME;`
I have a table in informix (Version 11.50.UC4) called NextRecordID with just one column called id and it will have one row. What I want to do is copy this value into another table. But don't want my query to fail if this table does not exist. Something like
if table NextRecordID exists
then insert into sometable values ('NextRecordID', (select id from NextRecordID))
else insert into sometable values ('NextRecordID', 1)
I ended up using the below SQL query. Its not ANSI SQL but works the informix server I am using.
insert into sometable values ('NextRecordID',
select case (select 1 from systables where tabname='nextrecordid')
when 1 then (select nextid from nextrecordid)
else (select 1 from systables where tabname='systables') end
from systables where tabname='systables');
What is happening here is within insert query I get the value to be inserted by using select query. Now that select query is interesting. It uses case statement of Informix. I have written a select query to check if the table nextrecordid exists in systables and return 1 if it exists. If this query returns 1, I query the table nextrecordid for the value or else I wrote a query to return the default value 1. This work for me.
You should be able to do this by checking the systables table.
Thank you for including server version information - it makes answering your question easier.
You've not indicated which language(s) you are using.
Normally, though, you design a program to expect a certain schema (certain tables to be present), and then fail - preferably under control - if those tables are not present. Also, it is not clear whether you would get into problems because of repeated execution of the second INSERT statement. Nor is it clear when the NextRecordID table is updated - presumably, once the value has been used, it must be updated.
You should look at SERIAL (BIGSERIAL) and see whether that is appropriate for you.
You should also look at whether a SEQUENCE would be appropriate to use here - it certainly looks rather like it might be applicable.
As Adam Hughes points out, if you want to check whether the NextRecordID table is present in the database, you would look in the systables table. Be aware, though, that your search will need to be against an all lower-case name (nextrecordid).
Also, MODE ANSI databases complicate life - you have to worry about the table's owner (because there could be multiple tables called nextrecordid in a MODE ANSI database). Most likely, you don't have to worry about that - any more than you are likely to have to worry about delimited identifiers for table "someone"."NextRecordID" (which is a different table from someone.NextRecordID).