When using the MySQL REPLACE statement to update another table based on a source table, we have the option of using a SELECT statement to select the whole table (or part of it) or the TABLE statement to select the whole table.
Below is an example of refreshing a table from another one using the TABLE statement with the REPLACE statement.
Generally speaking, most tables we create in MySQL should have a primary key. A primary key is one or more columns that have been configured as the unique identifier for the table.
We usually create the primary key when creating the table, but we can also add a primary key to an existing table (assuming it doesn’t already have one).
Occasionally it might not be immediately apparent which column is the primary key for a given table. Or it might not be apparent whether or not the table has a composite primary key. Fortunately it’s easy enough to find out.
Below are five ways to get the primary key column/s from an existing table in MySQL.
Continue readingIn MySQL we can use the information_schema.key_column_usage table to get information about the keys in a table. This table returns one row for each column that is constrained as a key.
We can use this table to find out which column/s is the primary key of a given table and to return any foreign keys from the table.
Continue readingIn MySQL there’s a system variable called sql_quote_show_create which is used to determine whether or not to use quoted identifiers when using the SHOW CREATE TABLE and SHOW CREATE DATABASE statements.
By default sql_quote_show_create is set to 1, which means that quotes/backticks will be generated whenever these statements are run, but we can also change this value to 0 in order to disable quotes.
Below are examples of checking this variable and setting it.
Continue readingIf you’re getting an error that reads “Failed to generate invisible primary key. Column ‘my_row_id’ already exists“, it’s probably because you’re trying to create a table without a primary key, and you’ve named a column my_row_id.
When your system has generated invisible primary keys (GIPKs) enabled, and you create an InnoDB table without explicitly defining a primary key, MySQL automatically creates an invisible column called my_row_id and creates a primary key constraint against it. This is called a generated invisible primary key.
However, if you name one of your columns my_row_id, then MySQL can’t create the GIPK due to the column already existing.
There are several ways to go about fixing this issue.
Continue readingMySQL allows us to easily add a primary key constraint to an existing column. We can use the ALTER TABLE statement to achieve this outcome.
We can also use the ADD PRIMARY KEY clause, which can be useful for adding composite primary keys.
MySQL supports invisible columns (from MySQL 8.0.23), which means we can control whether a column is considered visible or invisible.
Invisible columns are hidden from queries that use the asterisk wildcard (*) in their SELECT list, but can be selected by explicitly naming the column.
We can create invisible columns when creating a table. We can also add invisible columns to existing tables. And we can modify existing columns to be invisible (and vice-versa).
Continue readingIf you’re using the REPLACE statement to replace rows in MySQL, but it’s inserting duplicate rows instead, it could be due to one of the following reasons.
If you’re getting an error that reads something like “Multiple primary key defined” in MySQL, it’s probably because you’re defining more than one primary key to a table.
A table can only have one primary key in MySQL.
It’s possible that you were trying to create a composite primary key (i.e. a primary key consisting of multiple columns), in which case, you can fix the issue with a simple modification to your code.
Otherwise, you will need to decide which column should be the primary key for the table and only apply the PRIMARY KEY definition to that column.
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.