A composite primary key is a primary key that consists of two or more columns. Together they will always provide a unique value within the table. In other words, the combination of both values will be unique across all rows – no two rows will share the same combined value.

In MySQL we can create a composite primary key with the PRIMARY KEY clause of the CREATE TABLE statement. We can also use the ADD PRIMARY KEY statement to add a composite primary key to an existing table that doesn’t already have a primary key.

Primary keys are a fundamental part of relational database management systems (RDBMSs). They help us to maintain data integrity.

We can create primary keys when we create the table, or we can add one later.

When we create the primary key with the table, we have the option of defining the key within the actual column definition, or as a separate clause after all column definitions.

In MySQL we can use the REPLACE statement to replace data in an existing table without inserting a new row. When we do this, we have a choice of syntax when it comes to selecting the row to replace.

One option (and probably the most common option) is to use the VALUES clause. Another option is to use the SET clause.

Below is an example of using the SET clause when using MySQL’s REPLACE statement.

When we use the REPLACE statement in MySQL, we have the option of replacing the values of all columns in the row or just certain columns.

If we choose to replace just certain columns, we need to be aware of how this will affect the outcome. The “missing columns” in the title of this article refers to when we specify a value for just some, but not all columns.

If you’ve ever tried to copy a table with invisible columns in MySQL, you may have been baffled when you realised that none of the invisible columns were copied. Or, you may have copied them, but they ended up VISIBLE instead of INVISIBLE on the new table.

Fortunately, there’s an easy fix to this issue. Actually, there are at least a couple of ways we can approach it.