Join (SQL) and Update (SQL)

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Difference between Join (SQL) and Update (SQL)

Join (SQL) vs. Update (SQL)

A join clause in the Structured Query Language (SQL) combines columns from one or more tables into a new table. An SQL UPDATE statement changes the data of one or more records in a table. Either all the rows can be updated, or a subset may be chosen using a condition. The UPDATE statement has the following form: For the UPDATE to be successful, the user must have data manipulation privileges (UPDATE privilege) on the table or column and the updated value must not conflict with all the applicable constraints (such as primary keys, unique indexes, CHECK constraints, and NOT NULL constraints). In some databases, such as PostgreSQL, when a FROM clause is present, what essentially happens is that the target table is joined to the tables mentioned in the fromlist, and each output row of the join represents an update operation for the target table. When using FROM, one should ensure that the join produces at most one output row for each row to be modified. In other words, a target row shouldn't join to more than one row from the other table(s). If it does, then only one of the join rows will be used to update the target row, but which one will be used is not readily predictable. Because of this indeterminacy, referencing other tables only within sub-selects is safer, though often harder to read and slower than using a join. MySQL does not conform to ANSI standard.

Column (database)

In a relational database, a column is a set of data values of a particular type, one value for each row of a table.

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Database index

A database index is a data structure that improves the speed of data retrieval operations on a database table at the cost of additional writes and storage space to maintain the index data structure.

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Null (SQL)

In SQL, null or NULL is a special marker used to indicate that a data value does not exist in the database.

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Primary key

In the relational model of databases, a primary key is a specific choice of a minimal set of attributes (columns) that uniquely specify a tuple (row) in a relation (table).

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SQL

Structured Query Language (SQL) (pronounced S-Q-L; historically "sequel") is a domain-specific language used to manage data, especially in a relational database management system (RDBMS).

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Table (database)

In a database, a table is a collection of related data organized in table format; consisting of columns and rows.

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Where (SQL)

A WHERE clause in SQL specifies that a SQL Data Manipulation Language (DML) statement should only affect rows that meet specified criteria.

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