Oracle Reference: Various commands

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Check parameter Value
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SELECT * FROM V$PARAMETER
WHERE UPPER(NAME) LIKE '%UNDO%'

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Create Database Link
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CREATE DATABASE LINK my_link CONNECT TO user IDENTIFIED BY password USING 'connect_string';

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DBMS_OUTPUT.PUT_LINE
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To add empty line:
DBMS_OUTPUT.PUT_LINE(CHR(10));

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Check Oracle Instance name.
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SELECT * FROM V$INSTANCE
Or
SELECT * FROM GLOBAL_NAME;

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Update multiple columns

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UPDATE MY_TABLE
   SET COL_1 = VAL_1, 
       COL_2 = VAL_2
 WHERE COL_X = VAL_X;


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Add Constraint

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For Primary Key:

ALTER TABLE MY_TABLE ADD PRIMARY KEY  (col_a,col_b);
or
ALTER TABLE MY_TABLE ADD CONSTRAINT MY_TABLE_PK PRIMARY KEY  (col_a,col_b);
For Unique Constraint:
ALTER TABLE MY_TABLE ADD CONSTRAINT MY_TABLE_UQ UNIQUE  (col_a,col_b);

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INSERT INTO FROM SELECT

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To populate one table from another table

INSERT INTO TABLE_A(col1, col2, col3)
SELECT col1, col2, col3 FROM TABLE_B;