Solved: update sql server from another table

Updating SQL Server from another table is a task that can be common in the day-to-day task of a developer. When maintaining a database, you may encounter situations where you have to update some or all of the data in a field/column based on data from another table. This process can be made simple and efficient with the right use of SQL commands and logic.

A typical scenario is when you have two tables with similar columns but different data and you have to make one table’s data match the other. SQL gives you the tools to achieve this, but it requires a keen understanding of the UPDATE and SELECT statements, and how to use them together. Let’s explore how exactly this can be achieved.

Understanding SQL UPDATE and SELECT Statements

First, it’s important to clarify what the SQL UPDATE and SELECT statements are.

UPDATE statement is used to modify existing records in a table. 
The structure is: 
UPDATE table_name SET column1 = value1, column2 = value2, ... WHERE condition;

SELECT statement is used to select data from databases. 
The structure is: 
SELECT column1, column2, ... FROM table_name WHERE condition;

The UPDATE statement modifies the data of one or more records in a table. Either all the rows can be updated, or a set of rows which satisfies the given condition is updated.

The SELECT statement on the other hand, is used to fetch data from a database and it can be used to select data from one or more tables.

Updating A Table From Another Table

To update a table from another table in SQL, we leverage the combination of the UPDATE and SELECT commands. Here’s a general syntax:

UPDATE table1 SET table1.target_column=table2.target_column FROM table1, table2 WHERE table1.id=table2.id;

Let’s break down the process step-by-step:

• The UPDATE command is used, specifying the table to update.
• We then set the column that we want to update in table1. This comes after the SET keyword.
• We then set this column equal to the column in table2 that we want to take data from.
• Finally, we use the WHERE clause to specify the condition that needs to be met. In this case, we specify that we’re matching rows where the IDs in each table are the same.

Considerations and Warnings

While the process of updating a table from another table is relatively straight-forward with SQL, there are certain precautions and best practices you should observe:

• Always Backup: Before performing any UPDATE operations, especially those involving other tables, always make sure to have a recent backup of your databases in case any unforeseen problems occur.
• Performance: Try to limit the number of rows you’re updating at once, especially on large production databases. Updates on large tables can run for a long time and can potentially lock up a database.
• Test First: Before running the update, you can run a SELECT statement first to make sure the records selected are the ones you want to update.

With these principles and tips in mind, you can effectively update your SQL server from another table in a safe and controlled manner.