Learn about SQL Operators and Update Statements

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SQL update statements allow programmers to update data within an existing table that is part of a database. It can be used to update a single column or multiple columns at once. 

Included Pieces of Coding 

To make full use of the update statements associated with SQL operators, consider the following pieces of coding that can or should be incorporated. 

SET

Where you set the new values with an SQL operator. 

WHERE 

Indicates which rows and/or columns will receive the update for your SQL operator. If you do not use the WHERE clause in your coding, the corresponding columns in each row are updated, making the inclusion of the WHERE clause essential.

Syntax for Single vs. Multiple Tables 

The SQL update statement’s syntax can be for a single table or multiple tables. Take a closer look at the previously mentioned syntax and how it applies to your SQL operators in each situation. 

Single-Table Syntax

Updates the columns of your existing rows to include new values. This version of the UPDATE requires you to name the table. 

Use the SET clause to indicate the columns that you want to modify along with their desired values. 

Use the DEFAULT keyword to return a column to its default value, or include expressions as values within the SET clause of your SQL operators. 

Use the WHERE clause to indicate which rows to update. Otherwise, every row will update. 

Use the ORDER BY clause to view the rows in the order you indicate. 

Use the LIMIT clause to limit the number of rows that will be updated to a specified number. 

Multiple-Table Syntax

The use of various syntax components and SQL operators is similar for single- and multiple-table update statements. 

Use of UPDATE to update the rows in every table that you list within your table-references, provided it meets the required conditions. Every row that meets the conditions will only update one time, even if it meets the required conditions more than once. 

You cannot use LIMIT or ORDER BY with multiple-table syntax. 

Partitioned Tables

Partitioned tables are another scenario you will encounter with SQL operators and update statements. If you are handling partitioned tables, include the PARTITION option for both single- or multiple-table formats. This will create a list of at least one partition and/or sub-partition and will only check the partitions and sub-partitions that are on that list for matches. It does not matter if rows outside those elements meet your where_condition; they will not be updated according to the SQL operator rules. 

There may be times when you use PARTITION with an UPDATE and the result does not produce any updates since no rows in your sub-partitions or partitions that you listed meet the where_condition. However, this is still considered to be a successful statement. 

In situations where you are using REPLACE and INSERT statements, using a PARTITION and having no listed partitions or sub-partitions match your where_statement does not lead to a successful statement. This is an important distinction to consider when using SQL operators. 

Modifiers Supported

Using the UPDATE statement also gives you the ability to use other SQL operators as modifiers, each with its own effects.

IGNORE

Including this modifier results in the update statement continuing without aborting regardless of whether errors occur as it updates. It will not update rows that have duplicate-key conflicts, provided they occur with a unique key value. 

If the update statement updates a row to a value that will lead to errors converting the data, the update will change the value to be closest to the valid value. 

A Note on UPDATE IGNORE Statements

As you plan your SQL operator use, keep in mind that UPDATE IGNORE statements will be flagged as unsafe in situations where statement-based replication is desired. This includes situations where you include a clause for ORDER BY. 

The reason for this flagging comes down to the fact that the SQL operator rules state that the rows are ignored in the same order that they are updated. 

If you use an UPDATE IGNORE statement, you will see a warning. If you use statement-based mode, this warning will occur in the error log. If you use MIXED mode, it will appear in the binary log with the row-based format. 

LOW_PRIORITY

This modifier will delay your UPDATE from executing until there are no longer other clients that are reading from the relevant table. This particular modifier only has an impact if your storage engine relies on table-level locking. Examples include MERGE, MEMORY and MyISAM, where you would also use SQL operators. 

Required Privileges

You should also be familiar with the privileges required for SQL operators related to UPDATE statements. 

Essentially, you only need the privilege to create UPDATE statements for those columns that receive updates via the statement. If a column will be read without any modifications or updates, you do not need to include the UPDATE privilege for it; SELECT is enough. 

Evaluation Order

In the case of a single-table UPDATE assignment, the SQL operator code will usually evaluate them from left to right. However, with multiple-table updates, never assume that the assignments will occur in a given order, regardless of your SQL operators. 

Setting Values

MySQL notices when you set a column value. It will not be updated via the UPDATE SQL operator. 

Explicit Updates

If you explicitly update a generated column, it can only have the value of DEFAULT. 

NULL and NOT NULL

There may be times when you want to set the NOT NULL operator to NULL. If you have enabled strict SQL mode for your operators and change this setting after the system declares the column NOT NULL, you will notice an error. 

If you do not have strict SQL mode enabled, then the column in question will be set to the column data type’s implicit default value. Additionally, you will notice an increase in the warning count. Numeric types have an implicit default value of 0, while “zero” values are the implicit default for time and date types and the (‘ ‘) empty string is the implicit default for a string type. 

Conclusion

SQL update statements are an excellent way to update the information in your tables, whether you have one or many. You can combine the SQL operators you learn about in a web coding bootcamp in your UPDATE statement. 

*Please note, these articles are for educational purposes and the topics covered may not be representative of the curriculum covered in our boot camp. Explore our curriculum to see what you’ll learn in our program.

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