SQL IF / ELSE in SQLite

Introduction to Conditional Statements

SQL is a declarative language that allows us to specify what data we want to retrieve, insert, update, or delete. However, it does not have built-in conditional statements like IF and ELSE. This limitation can make certain operations more complicated.

In this article, we will explore how to achieve similar functionality in SQLite using various techniques.

SQL IF / ELSE Statement in MySQL

For those familiar with MySQL, let’s take a look at the syntax for an IF/ELSE statement:

IF NOT EXISTS (SELECT mtrNo from tblMeter where mtrNo = '0000033')
    BEGIN
        insert into tblMeter (mtrNo) values ('0000033')
    END
ELSE
    BEGIN 
        update tblMeter set Name = 'A-15' where mtrNo = '0000033'
    END

This syntax uses a combination of the NOT EXISTS and INSERT INTO statements to check if the row already exists in the table. If it does not exist, the statement inserts a new row; otherwise, it updates the existing row.

SQL IF / ELSE Statement in SQLite

Unfortunately, SQLite does not have an exact equivalent of MySQL’s IF/ELSE statement. However, we can achieve similar behavior using different techniques.

Using the UPSERT Statement

SQLite provides the UPSERT statement, which is similar to the INSERT OR REPLACE statement used in other databases. The syntax for the UPSERT statement is as follows:

insert into tblMeter AS ti
       (rowid, mtrNo)
       SELECT COALESCE(t1.rowid, (SELECT max(tx.rowid) FROM tblMeter AS tx) + 1, 1) AS rid,
              val.mtrNo
              FROM (SELECT '0000033' AS mtrNo) AS val
                   LEFT JOIN tblMeter AS t1
                   ON val.mtrNo == t1.mtrNo
WHERE true --avoid parsing ambiguity
ON CONFLICT (rowid)
DO UPDATE SET Name = 'A-15'; --Only rows with matching mtrNo will be updated

This statement first tries to insert a new row into the table. If the row already exists, SQLite updates the existing row instead of replacing it.

Using the INSERT OR REPLACE Statement

Another way to achieve similar behavior is by using the INSERT OR REPLACE statement:

INSERT OR REPLACE INTO tblMeter
       (rowid, mtrNo, Name)
       SELECT COALESCE(t1.rowid, (SELECT max(tx.rowid) FROM tblMeter AS tx) + 1, 1) AS rid,
              val.mtrNo,
              val.Name
              FROM (SELECT '0000033' AS mtrNo, 'A-15' AS Name) AS val
                   LEFT JOIN tblMeter AS t1
                   ON val.mtrNo == t1.mtrNo;

This statement will delete any existing rows with the same mtrNo before inserting or replacing them. This is a more drastic operation than using the UPSERT statement, so use it carefully.

Choosing Between INSERT OR REPLACE and UPSERT

When deciding between using the INSERT OR REPLACE statement or the UPSERT statement, consider the following factors:

• Data Integrity: The UPSERT statement preserves data integrity better because it only updates existing rows instead of replacing them. This is particularly important when working with tables that have unique constraints.
• Complexity: The UPSERT statement can be more complex to use than the INSERT OR REPLACE statement, especially in cases where there are multiple columns involved.

In general, if you need to update existing data while preserving data integrity, choose the UPSERT statement. If you’re working with tables that have a primary key constraint and want to replace existing rows entirely, use the INSERT OR REPLACE statement.

Conclusion

SQLIF/ELSE statements are not built-in in SQLite, but there are several ways to achieve similar functionality using different techniques. The UPSERT statement is one such approach that preserves data integrity better than the INSERT OR REPLACE statement.

While it may seem more complicated at first, mastering the UPSERT statement and learning its nuances can help you write more effective SQL code and handle complex database operations more efficiently.

Whether you choose to use INSERT OR REPLACE or UPSERT depends on your specific requirements. Always consider factors such as data integrity, complexity, and performance when making your decision.

In this article, we explored how to create an IF/ELSE statement-like behavior in SQLite using the UPSERT statement and the INSERT OR REPLACE statement.

Last modified on 2023-07-11