How to Merge Two Databases in Mysql

Definition of MySQL Merge

MySQL Merge is a MySQL statement which allows us to update records in a specific table on the basis of values that matches from another database table. The MySQL Merge query command is responsible to perform three major query operations at the same time. Suppose, when we apply the CRUD operation commands such as INSERT, DELETE and UPDATE distinctly in our database queries then, we must have to build up three different MySQL statements so that the data in the destination table can be modified using the corresponding rows from the database source table.

For this the users should have SELECT, INSERT, DELETE and UPDATE privilegeson the database tables withwhich you will map to a MERGE table.

Syntax of MySQL Merge

The succeeding code of structure defines an elementary syntax for MySQL Merge statement query:

Merge TargetTableName USING SourceTableName
ON Merging_Condition
WHEN MATCHED
THEN Update_Query
WHEN NOT MATCHED
THEN Insert_Query
WHEN NOT MATCHED BY SOURCE
THEN DELETE;

Let us discuss the syntax as follows:

• Initially, we will define the target and source database tables in the MERGE clause query.
• Then, we will add the merging condition that decides how the table rows from the table source will be matched to the table rows from the table target one. This can be said as similar to a JOIN condition used in the JOIN clause. Normally, we will apply the key columns that can be either UNIQUE Key or PRIMARY Key for the purpose of matching.
• After that, this merge condition provides three simple states to query as below:
  • Matched: It defines the table rows that will match the Merge condition and for this we need to alter the table rows columns in the table target with the values form the table source. We use the UPDATE statement here.
  • Not Matched: It denotes the table rows from the source table which do not have any equivalent rows present in the target table. In this type of case, we should supplement the rows from the source table to the defined target table. Remember that NOT MATCHED can also be noted as NOT MATCHED BY TARGET. We will use the INSERT query here.
  • Not Matched by Source: It defines the table rows in the given target table which do not have any equivalent rows in the particular source table. We have to apply the match condition to remove or delete rows from the given target table if we want to coordinate the target table with the records from the source one. We will use the DELETE query here.

How does Merge Work in MySQL?

Assume that we have two database tables known as source table and target table. Using these tables we need to perform matching of values from the source table and update the target table. It provides three different cases described as follows:

• The source table may include few rows that are not present in the target table. Therefore, in this case we can use the INSERT command to input rows into the target table that are found in the source table.
• The target table may include few rows that are not present in the source table. Therefore, in this case we can use the DELETE command to remove rows from the target table that are not found in the source table.
• In this case, we will find some table rows in the source table containing similar keys as that of target table rows. However, in the non-key table columns these table rows contain different values. Here, we will update the table rows in the target table with the approaching values from the source table.

A MERGE table is also recognized as anMRG_MyISAM table in the database, which denotes anassembly of comparableMyISAM tables which can be used as a single table. We can perform only SELECT, UPDATE and DELETE procedures on this group of tables. But suppose we want to drop the MERGE table, then after dropping only the MERGE specifications will be released. Here, the tables created comprises of identical table column and info of key because with columns packed inverselyor keys having another order, we cannot apply MERGE over that tables. In MySQL, merge can be performed with UNION, INSERT, or even using JOINS on two or more tables to combine the data values on the basis of UNIQUE key or PRIMARY key.

Examples of MySQL Merge

We will create two tables in the database named Products and Products_Info that will contain information of products.

CREATE TABLE Products (ProductID INT PRIMARY KEY, Product_Name VARCHAR(255) NOT NULL, Cost INT NOT NULL);

Inserting some items as:

INSERT INTO Products(ProductID, Product_Name, Cost) VALUES
(1,'Parle G',100),
(2, 'Maggie', 112),
(3, 'GoodDay Biscuit', 150);

View the table Products:

SELECT * FROM Products;

Output:

Again,

CREATE TABLE Products_Info (ProductID INT PRIMARY KEY, Product_Name VARCHAR(255) NOT NULL, Cost INT NOT NULL);

Inserting some items as:

INSERT INTO Products_Info(ProductID, Product_Name, Cost) VALUES
(1,'Parle G', 100),
(2, 'Maggie', 112),
(3, 'GoodDay Biscuit', 115),
(4, 'Nestle Coffee', 125),
(5, 'TATA Tea', 80);

View the table Products:

SELECT * FROM Products_Info;

Output:

Now, with the values from the Products_Info table which is as source table by using the MERGE statement to update data to the Products table as target table we will use this below query in SQL because MySQL version may not support MERGE so, we will write code for it also to demonstrate:

MERGE Products t
USING Products_Info s
ON (s.ProductID = t.ProductID)
WHEN MATCHED
THEN UPDATE SET
t.Product_Name = s.Product_Name,
t.Cost = s.Cost
WHEN NOT MATCHED BY TARGET
THEN INSERT (ProductID, Product_Name,Cost)
VALUES(s.ProductID,s.Product_Name,s.Cost)
WHEN NOT MATCHED BY SOURCE
THEN DELETE;

Output:

In MySQL, MERGE is not supported and we apply INSERT…..ON DUPLICATE KEY UPDATE where MySQL performs update on old tables values based on the new ones. Hence, for MySQL we can follow the below queries to combine two tables:

INSERT IGNORE INTO Products SELECT * FROM Products_Info;

Output:

SELECT * FROM Products UNION DISTINCT SELECT * FROM Products_Info;

Output:

SELECT * FROM Products INNER JOIN Products_Info ON Products.ProductID = Products_Info.ProductID;

Output:

Conclusion

• MySQL Merge solves a lot of problems like it helps in managing log tables set easily, provides extra speed, can do proficiently the searches and repair operations, and more.
• The MySQL Merge is useful to map various files to a single one instantly and implements additional file descriptors. But in Merge tables, you cannot use the REPLACE query.

Recommended Articles

This is a guide to MySQL Merge. Here we also discuss the definition and how does merge work in mysql? along with different examples and its code implementation. You may also have a look at the following articles to learn more –

1. MySQL having
2. MySQL BLOB
3. MySQL today()
4. MySQL Create Function

How to Merge Two Databases in Mysql

Source: https://www.educba.com/mysql-merge/

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