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FOREIGN KEY (child_col1, child_col2, child_col_n)
REFERENCES parent_table (parent_col1, parent_col2, parent_col_n)
ON DELETE SET NULL
[ ON UPDATE { NO ACTION | CASCADE | SET NULL | SET DEFAULT } ]
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DELETE FROM user WHERE username IS NULL;
To Delete the Rows with the Null values if present in any specific Columns.
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CREATE TBALE parent_table_p (
id INT PRIMARY KEY
)
CREATE_TABLE child_table_c(
id INT PRIMARY KEY,
parent_id INT,
FOREIGN KEY (parent_id) REFERENCES parent_table_p(id) ON DELETE SET NULL
)