- Sql Run Multiple Queries
- Select For Update Mysql Example Query
- Mysql Update Table From Select
- Mysql Update Query Example
What are sub queries?
A list of commonly used MySQL queries to create database, use database, create table, insert record, update record, delete record, select record, truncate table and drop table are given below. 1) MySQL Create Database. MySQL create database is used to create database. More Details. 2) MySQL Select/Use Database. You can use a subquery within any of the following statements: SELECT, INSERT, UPDATE, DELETE, SET, or DO. For example, you could use a subquery inside an INSERT statement to insert data based on the results of a subquery.
A sub query is a select query that is contained inside another query. The inner select query is usually used to determine the results of the outer select query.
Let's look into the sub query syntax -
A common customer complaint at the MyFlix Video Library is the low number of movie titles. The management wants to buy movies for a category which has least number of titles.
You can use a query like
It gives a result
Let's see how this query works
The above is a form of Row Sub-Query. In such sub-queries the , inner query can give only ONE result. The permissible operators when work with row subqueries are [=, >, =, <=, ,!=, ]
Let's look at another example ,
Suppose you want Names and Phone numbers of members of people who have rented a movie and are yet to return them. Once you get Names and Phone Number you call them up to give a reminder. You can use a query like
Let's see how this query works
In this case, the inner query returns more than one results. The above is type of Table sub-query.
Till now we have seen two queries , lets now see an example of triple query!!!
Suppose the management wants to reward the highest paying member.
We can run a query like
The above query gives the following result -
Sub-Queries Vs Joins!
When compare with Joins , sub-queries are simple to use and easy to read. They are not as complicated as Joins
Hence there are frequently used by SQL beginners.
But sub-queries have performance issues. Using a join instead of a sub-query can at times give you upto 500 times performance boost.
Given a choice, it is recommended to use a JOIN over a sub query.
Sub-Queries should only be used as a fallback solution when you cannot use a JOIN operation to achieve the above
Summary
- Subqueries are embedded queries inside another query. The embedded query is known as the inner query and the container query is known as the outer query.
- Sub queries are easy to use, offer great flexibility and can be easily broken down into single logical components making up the query which is very useful when Testing and debugging the queries.
- MySQL supports three types of subqueries, scalar, row and table subqueries.
- Scalar sub queries only return a single row and single column.
- Row sub queries only return a single row but can have more than one column.
- Table subqueries can return multiple rows as well as columns.
- Subqueries can also be used in INSERT, UPDATE and DELETE queries.
- For performance issues, when it comes to getting data from multiple tables, it is strongly recommended to use JOINs instead of subqueries. Sub queries should only be used with good reason.
UPDATE statement is used for updating existing rows of a table. It has following syntax where values are set for each column to be updated.
You can run UPDATE statements in MySQL command-line, in a GUI tool or with mysqli_query() in a PHP script.
For this article, we are going to use `employee` table mentioned below. SQL commands for creating the table, inserting data and, truncating are available here.
id | first_name | last_name | job_title | salary | notes |
---|---|---|---|---|---|
1 | Robin | Jackman | Software Engineer | 5500 | |
2 | Taylor | Edward | Software Architect | 7200 | |
3 | Vivian | Dickens | Database Administrator | 6000 | |
4 | Harry | Clifford | Database Administrator | 6800 | |
5 | Eliza | Clifford | Software Engineer | 4750 | |
6 | Nancy | Newman | Software Engineer | 5100 | |
7 | Melinda | Clifford | Project Manager | 8500 | |
8 | Harley | Gilbert | Software Architect | 8000 |
We recommend truncating the table and re-entering data after each example so that you can identify the change clearly (by comparing with original table shown above).
Result sets listed in this article (except for amending values section) have been generated in that way and by executing SELECT statements (SELECT * FROM `employee`) in command-line.
Updating Rows that Match Conditions
Following is how to update the salary of an employee whose ID is 3.
It’s optional to have a WHERE clause in UPDATE statements. But not having it can lead to critical errors. For an example, in above query, if WHERE clause was omitted, salary will be set to 6500 for all the employees.
If the company wanted to set the minimum salary to 5500, following query can do the necessary update.
- When executing above query, three rows become eligible to be updated. But MySQL updates only two rows since one’s value is already 5500.
- MySQL returns 2 (number of rows affected) which will be the return value of mysqli_affected_rows() if you ran the query in a PHP script.
Above query could have been written as below by having < operator in place of <= operator which will select only two rows to be updated.
You can specify more than one conditions in WHERE clause using AND, OR and IN() as mentioned in article on SELECT statement.
Making Amendments to Existing Values
Sometimes you may not want to replace existing values and only want to make adjustments to existing values. For an example, if you want to increase the salary by 200 of the employee with ID 1, you can use following query.
Sql Run Multiple Queries
Following query appends a note to the existing `notes` value after a period and a space. If the current value is NULL, separator is omitted and only the new note is added (it's the behavior of CONCAT_WS() function).
If the column type is numeric, you can use arithmetic operators (+, -, * and /) directly and if it is string, you have to use string functions like CONCAT_WS() and CONCAT().
Setting the Order of Update
Select For Update Mysql Example Query
MySQL doesn't have a guaranteed order for updates. In cases where order is important, you can specify it like below.
In above query, we increment all the IDs by one and it starts incrementing from the last row (8 becomes 9). If it didn't have this order and MySQL tried to do the increment from first row (making 1 into 2), it will throw an error since ID number 2 already exists (`id` has to be unique since it's the primary key).
Limiting Number of Rows to Update
Mysql Update Table From Select
Drivers sis 330 mirage igp para windows 7 download. You can make MySQL increment the ID of only the last two rows by specifying a LIMIT clause as below.
Using MySQL Update Statement in PHP
Mysql Update Query Example
Following PHP script shows how to use an update statement in PHP. Two commented out queries correspond to first two conditions of the if-else block. You can try out all three queries by commenting out two queries at a time.