MySQL has a SHOW CREATE DATABASE statement that shows the CREATE DATABASE statement that would be used to recreate the database.
We can use the SHOW CREATE DATABASE statement to create an identical database on another server, or to create a similar database on the same server (while changing the name).
We can alternatively use SHOW CREATE SCHEMA to do the same thing (SHOW CREATE SCHEMA is a synonym for SHOW CREATE DATABASE).
If 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.
In MySQL, we can use the SHOW CREATE TABLE statement to produce a CREATE TABLE statement from an existing table.
This enables us to generate a script that we can use to recreate the table on another database, or on the same database at a later date. Or we could use it to create another similar table. We would need to modify the code accordingly when doing this, but it could give us a good starting point.
We can also use SHOW CREATE TABLE for a quick way to take a look at the table’s structure, and check for things like data types, primary keys, foreign keys, etc.
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.
MySQL provides the sql_quote_show_create system variable, which allows us to specify whether or not to include quotes around identifiers when using the SHOW CREATE TABLE and SHOW CREATE DATABASE statements.
By default, this variable is set to 1, which means that quotes/backticks will be generated whenever these statements are run.
However, we might not always want quotes or backticks around the object names, and so we can change the value of this variable to disable quoted identifiers.
Continue readingA 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.
In MySQL we can use the DO statement if we want to execute an expression without getting a result set.
DO works in a similar way to the SELECT statement, but without returning a result set like SELECT normally would. DO can also be slightly faster, given it doesn’t return a result set.
This could be useful in stored functions or triggers that don’t allow us to run statements that return a result set.
Continue readingPrimary 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.
Continue readingIn 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.
If you’re getting an error in MySQL that reads something like “Duplicate entry ‘1’ for key ‘person.PRIMARY’” when trying to add a primary key to an existing table, it’s probably because the table already contains duplicate values in the column you’re trying to add the primary key to.
When we add a primary key to an existing table, we need to make sure that the column/s that we’re adding the primary key to contains unique values across all of its rows.
One way to fix this issue is to create a composite primary key (one that’s defined across multiple columns). Another way to fix it is to change the values so that they’re all unique. Another option is to apply the primary key to a different column altogether (i.e. one that contains unique values).
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