Data dictionary queries
1. Check if a table exists in the current database schema
A simple query that can be used to check if a table
exists before you create it. This way you can make your create table script
rerunnable. Just replace table_name with actual table you want to check. This
query will check if table exists for current user (from where the query is
executed).
SELECT table_name
FROM user_tables
WHERE table_name = 'TABLE_NAME';
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2. Check if a column exists in a table
Simple query to check if a particular column exists
in table. Useful when you tries to add new column in table using ALTER TABLE
statement, you might wanna check if column already exists before adding one.
SELECT column_name AS FOUND
FROM user_tab_cols
WHERE table_name = 'TABLE_NAME' AND column_name = 'COLUMN_NAME';
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3. Showing the table structure
This query gives you the DDL statement for any
table. Notice we have pass ‘TABLE’ as first parameter. This query can be
generalized to get DDL statement of any database object. For example to get DDL
for a view just replace first argument with ‘VIEW’ and second with your view
name and so.
SELECT DBMS_METADATA.get_ddl ('TABLE', 'TABLE_NAME',
'USER_NAME') FROM DUAL;
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4. Getting current schema
Yet another query to get current schema name.
SELECT SYS_CONTEXT ('userenv', 'current_schema') FROM DUAL;
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5. Changing current schema
Yet another query to change the current schema.
Useful when your script is expected to run under certain user but is actually
executed by other user. It is always safe to set the current user to what your
script expects.
ALTER SESSION SET CURRENT_SCHEMA = new_schema;
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