Question

The purpose of this is to copy some rows from one environment to another without overwriting existing rows.

Sample DB:

INSERT INTO `school` (school_id,name) VALUES (15,'Middle');
INSERT INTO `class` (class_id,school_id,name) VALUES (12,15,'Sample');

The idea is school_id and class_id are auto-increments and class has a Foreign Key link back to school. But I want to dump just these rows and insert them into another database that already has a school_id of 15.

It might be something that could look like:

INSERT INTO `school` (name) VALUES ('Middle');
INSERT INTO `class` (school_id,name) VALUES (LAST_INSERT_ID(),'Sample');

But that would just be for this simple example. Imagine if I had 50 classes, 25 students in each, and a few hundred grades for each student/class combo. You could see how the LAST_INSERT_ID() might not work without storing it in a series of variables.

What would be the proper tool to do this kind of operation? Can mysqldump do anything this smart?

Answer 1

You can do this:

• Find MAX school_id in the target school table –

  SELECT MAX(school_id) INTO @max_school_id FROM school;

• Change all school_id values in source tables (school, class) – add MAX school_id from the previous point –

  UPDATE school SET school_id = school_id + @max_school_id + 1;

It might be very usefull to add ON UPDATE CASCADE action to the foreign key, it will help to change school_id in the child table automatically, e.g. –

ALTER TABLE class
  DROP FOREIGN KEY FK_name;
ALTER TABLE class
  ADD CONSTRAINT FK_name FOREIGN KEY (school_id)
    REFERENCES school(school_id) ON UPDATE CASCADE;

• Make dump and import.

---

Explanation and example:

Create source tables:

CREATE TABLE school(
  school_id INT PRIMARY KEY AUTO_INCREMENT,
  name VARCHAR(20)
);

INSERT INTO school (school_id, name) VALUES
  (1, 'Middle1'),
  (2, 'Middle2'),
  (3, 'Middle3'),
  (15, 'Middle');

CREATE TABLE class(
  class_id INT(11) NOT NULL,
  school_id INT(11) DEFAULT NULL,
  name VARCHAR(20) DEFAULT NULL,
  PRIMARY KEY (class_id),
  CONSTRAINT FK_class_school_school_id FOREIGN KEY (school_id)
  REFERENCES school (school_id) ON DELETE RESTRICT ON UPDATE CASCADE
)
ENGINE = INNODB;

INSERT INTO class (class_id, school_id, name) VALUES (11, 1, 'Sample1');
INSERT INTO class (class_id, school_id, name) VALUES (12, 15, 'Sample');

Create target tables:

CREATE TABLE school(
  school_id INT PRIMARY KEY AUTO_INCREMENT,
  name VARCHAR(20)
);

INSERT INTO school (school_id, name) VALUES
  (1, 'Top'),
  (2, 'Middle'),
  (3, 'Bottom'),
  (15, 'Top');

CREATE TABLE class(
  class_id INT(11) NOT NULL,
  school_id INT(11) DEFAULT NULL,
  name VARCHAR(20) DEFAULT NULL,
  PRIMARY KEY (class_id),
  CONSTRAINT FK_class_school_school_id FOREIGN KEY (school_id)
  REFERENCES school (school_id) ON DELETE RESTRICT ON UPDATE CASCADE
)
ENGINE = INNODB;

INSERT INTO class (class_id, school_id, name) VALUES (10, 2, 'Sample2');
INSERT INTO class (class_id, school_id, name) VALUES (12, 15, 'Sample');

Update source tables, increment id values:
We should update all unique values, in our case we have to update class_id in the class table and school_id in the school table.

Find max class_id for the TARGET class table

SELECT MAX(class_id) + 1000 FROM class; -- This will return => 1012

Increment all SOURCE class_id values class_id + 1012

UPDATE class SET class_id = class_id + 1012;

Find max school_id for the TARGET school table

SELECT max(school_id) + 1000 FROM school; -- This will return =>1015

Increment all SOURCE school_id values school_id + 1015

UPDATE school SET school_id = school_id + 1015;

That is all. We can dump source tables:

INSERT INTO school VALUES
  (1016, 'Middle1'),
  (1017, 'Middle2'),
  (1018, 'Middle3'),
  (1030, 'Middle');

INSERT INTO class VALUES
  (1023, 1016, 'Sample1'),
  (1024, 1030, 'Sample');

Now we can easily run this script against the target database.