Advanced queries are a part of every database administrator or developer job. Advanced queries must be handled delicately, because improperly coded SQL or poorly performing SQL can create bugs and application crashes. Advanced queries are typically used for reporting, joining multiple tables, nesting queries, and transaction locking. All of these concepts are covered in this article.
For example, in our sample table 'emp', if most of the queries fetch data using the column id, then it would be a wise decision to create an index for the id column. Upon creating indexes, MySQL will first search for ID in the created index and if none is found it will run the query on the table level. The SQL UPDATE Statement. The UPDATE statement is used to modify the existing records in a table. UPDATE table_name. SET column1 = value1, column2 = value2. WHERE condition; Note: Be careful when updating records in a table! Notice the WHERE clause in the UPDATE statement. 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. The SQL UPDATE Statement. The UPDATE statement is used to modify the existing records in a table. UPDATE table_name. SET column1 = value1, column2 = value2. WHERE condition; Note: Be careful when updating records in a table! Notice the WHERE clause in the UPDATE statement.
Using Aliases in Your Queries
Aliases let you create a shortcut name for different table options. With aliasing, you can use one letter for a table name instead of typing the full name throughout your entire query. Aliases make queries easier to read, and they make them faster to code for developers.
You have the option to alias either table columns or table names. Aliasing tables is probably the most common shorthand form in everyday queries, but some coders alias columns as well.
If you remember from previous chapters, we typed the full Customer table and column names when we queried more than one table with a WHERE clause. The above query is the same as the following query:
SELECT first_name, last_name, order_id
INNER JOIN Order on Customer.customer_id = Order.customer_id
WHERE Customer.customer_id = 1;
You need to specify a table name in the WHERE clause to avoid ambiguous column names. Both the Order table and the Customer table have a customer_id column. If you leave out the alias or full table name, the MySQL engine doesn't know which table to use in the WHERE clause. The result is an error from MySQL that indicates ambiguous column names.
To specify tables without being forced to type the full table name for every column, MySQL lets you use aliases. You use aliases with the AS command.
Let's first take a look at table aliasing. Here is an example query.
SELECT first_name, last_name, order_id
FROM Customer AS c
INNER JOIN Order AS o on c.customer_id = c.customer_id
WHERE c.customer_id = 1;
The result from this query is the following result set:
first_name, last_name, order_id
John Smith 1
Notice the AS keyword just before the alias name. We've used the letter 'c' to create an alias for the table Customer.
If you alias one table, it's best to alias the rest of the tables in your query. We aliased the table Order with the letter 'o.' If you compare the previous query with the one above it, you can see that the code is cleaner, shorter, and easier to read.
You can also alias column names. This is common when you have aggregate functions in your queries and need to give the results a column name. Let's take the following query as an example:
SELECT COUNT(customer_id) FROM Customer;
The above statement counts the number of customer records in your Customer database. Without an alias name, you rely on MySQL to generate the column name, and it becomes difficult to reference the results in your applications. To overcome this hurdle, you use aliases to define the column name.
Let's modify the previous example to use aliases:
SELECT COUNT(customer_id) AS customer_count FROM Customer;
Now, when we run the above query, the result is the following data set:
Aliasing column names makes it much easier to identify aggregate counts and functions within your queries.
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Aggregating Your Data
The COUNT function above is just one example of aggregating data. MySQL has numerous functions that make it easier to group, add, average, and count data.
There are five main MySQL internal functions, but you can create your own functions. The five aggregate functions include:
How do I update a table and set different values upon the condition evaluating to True.
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Update Mysql W3schools
For instance :
I have seen CASE expression and IF expression in Procedures and Functions but I want to use it in a simple update/select statement. Is it possible or am I expecting too much from this lovely open source database?ThinkCodeThinkCode
you might want to use
A is always a floating point value
> 0 and
Whilst you certainly can use MySQL's
IF() control flow function as demonstrated by dbemerlin's answer, I suspect it might be a little clearer to the reader (i.e. yourself, and any future developers who might pick up your code in the future) to use a
CASE expression instead:
Of course, in this specific example it's a little wasteful to set
A to itself in the
ELSE clause—better entirely to filter such conditions from the
UPDATE, via the
(The inequalities entail
A IS NOT NULL).
Or, if you want the intervals to be closed rather than open (note that this would set values of
1—if that is undesirable, one could explicitly filter such cases in the
WHERE clause, or else add a higher precedence
Though, as dbmerlin also pointed out, for this specific situation you could consider using
Sql Run Multiple Queries
Here's a query to update a table based on a comparison of another table. If record is not found in tableB, it will update the 'active' value to 'n'. If it's found, will set the value to NULL
Hope this helps someone else.