Why is a UNION on two numbers more efficient than their sum?

We want to ensure that there are no more than maxCount palettes at a location (with a certain type of inventory). In order to ensure that, we sum up locations with palettes on them or locations to which palettes will be located. Both statements work fine. However, my initial draft was not very performant. My advisor came up with a different draft which is much faster, but he could not explain why.

We want to ensure that there are no more than maxCount palettes at a location (with a certain type of inventory). In order to ensure that, we sum up locations with palettes on them or locations to which palettes will be located. Both statements work fine. However, my initial draft was not very performant. My advisor came up with a different draft which is much faster, but he could not explain why.

Slow statement:

 AND maxCount > ( 
    SELECT 
        trptoloc+atloc
    FROM
        (
            SELECT
                COUNT(DISTINCT loc2.locnam) AS atloc
            FROM
                loc_t loc2,
                pal_t pal,
                inv_t inv,
                art_t art
            WHERE
                loc.group = loc2.group
                AND   pal.locnam = loc2.locnam
                AND   inv.palnam = pal.palnam
                AND   inv.artid = art.artid
                AND   AND   art.feature = value
        ),
        (
            SELECT
                COUNT(DISTINCT loc2.locnam) AS trptoloc
            FROM
                trp_t trp,
                loc_t loc2,
                inv_t inv,
                art_t art
            WHERE
                trp.locnamezl = loc2.locnam
                AND   trp.palnam = inv.palnam
                AND   loc.group = loc2.group
                AND   inv.artid = art.artid
                AND   art.feature = value;
        )
)

Faster statement:

AND maxCount > (
 SELECT
 SUM(invpal)
 FROM
 (SELECT
    COUNT(distinct inv.palnam) AS invpal
    FROM
        inv_t inv,
        art_t art,
        pal_t pal,
        loc_T loc2
    WHERE
        inv.artid = art.artid
        AND   loc.group = loc2.group
        AND   pal.locnam = loc2.locnam
        AND   inv.palnam = pal.palnam
        AND   art.feature = value;
  UNION ALL
    SELECT COUNT(distinct inv.palnam) AS invpal
    FROM
        inv_t inv,
        art_t art,
        trp_t trp,
        loc_T loc2
    WHERE
        trp.locnamezl = loc2.locnam
        AND   trp.palnam = inv.palnam
        AND   loc.group = loc2.group
        AND   inv.artid = art.artid
        AND   art.feature = value;
)
)

Any hint or link that will help me understand the performance difference is appreciated!

An Introduction to Oracle SQL

An Introduction to Oracle SQL

An Introduction to Oracle SQL: A quick tour of the six principal clauses of the SELECT statement.

This course is a brief presentation of Oracle SQL.
We will install a version of Oracle Express Edition and Oracle SQL Developer for free.
We'll then look at the SELECT statement with its 6 main clauses (SELECT, FROM, WHERE, GROUP BY, HAVING and ORDER BY).
Following this, we'll look at what else you need to know, and resources (which range from FREE to very expensive) which will help you to learn it..

What you'll learn

  • Either Download and Install Oracle Express Edition and Oracle SQL Developer on your computer - for free
  • Write your own SELECT statements.
  • Learn the SELECT, FROM, WHERE, GROUP BY, HAVING and ORDER BY clauses.
  • Get an easy way to remember the order of these clauses.
  • Find out what more you need to know, and resources in how to learn it.

How To Manage an SQL Database

How To Manage an SQL Database

SQL databases come installed with all the commands you need to add, modify, delete, and query your data. This cheat sheet-style guide provides a quick reference to some of the most commonly-used SQL commands.

SQL databases come installed with all the commands you need to add, modify, delete, and query your data. This cheat sheet-style guide provides a quick reference to some of the most commonly-used SQL commands.

An SQL Cheat Sheet

Introduction

SQL databases come installed with all the commands you need to add, modify, delete, and query your data. This cheat sheet-style guide provides a quick reference to some of the most commonly-used SQL commands.

How to Use This Guide:

  • This guide is in cheat sheet format with self-contained command-line snippets
  • Jump to any section that is relevant to the task you are trying to complete
  • When you see <span class="highlight">highlighted text</span> in this guide’s commands, keep in mind that this text should refer to the columns, tables, and data in your own database.
  • Throughout this guide, the example data values given are all wrapped in apostrophes ('). In SQL, it is necessary to wrap any data values that consist of strings in apostrophes. This isn’t required for numeric data, but it also won’t cause any issues if you do include apostrophes.

Please note that, while SQL is recognized as a standard, most SQL database programs have their own proprietary extensions. This guide uses MySQL as the example relational database management system (RDBMS), but the commands given will work with other relational database programs, including PostgreSQL, MariaDB, and SQLite. Where there are significant differences between RDBMSs, we have included the alternative commands.

Opening up the Database Prompt (using Socket/Trust Authentication)

By default on Ubuntu 18.04, the root MySQL user can authenticate without a password using the following command:

sudo mysql


To open up a PostgreSQL prompt, use the following command. This example will log you in as the postgres user, which is the included superuser role, but you can replace that with any already-created role:

sudo -u postgres psql


Opening up the Database Prompt (using Password Authentication)

If your root MySQL user is set to authenticate with a password, you can do so with the following command:

mysql -u root -p


If you’ve already set up a non-root user account for your database, you can also use this method to log in as that user:

mysql -u user -p


The above command will prompt you for your password after you run it. If you’d like to supply your password as part of the command, immediately follow the -p option with your password, with no space between them:

mysql -u root -ppassword


Creating a Database

The following command creates a database with default settings.

CREATE DATABASE database_name;


If you want your database to use a character set and collation different than the defaults, you can specify those using this syntax:

CREATE DATABASE database_name CHARACTER SET character_set COLLATE collation;


Listing Databases

To see what databases exist in your MySQL or MariaDB installation, run the following command:

SHOW DATABASES;


In PostgreSQL, you can see what databases have been created with the following command:

\list


Deleting a Database

To delete a database, including any tables and data held within it, run a command that follows this structure:

DROP DATABASE IF EXISTS database;


Creating a User

To create a user profile for your database without specifying any privileges for it, run the following command:

CREATE USER username IDENTIFIED BY 'password';


PostgreSQL uses a similar, but slightly different, syntax:

CREATE USER user WITH PASSWORD 'password';


If you want to create a new user and grant them privileges in one command, you can do so by issuing a GRANT statement. The following command creates a new user and grants them full privileges to every database and table in the RDBMS:

GRANT ALL PRIVILEGES ON *.* TO 'username'@'localhost' IDENTIFIED BY 'password';


Note the PRIVILEGES keyword in this previous GRANT statement. In most RDBMSs, this keyword is optional, and this statement can be equivalently written as:

GRANT ALL ON *.* TO 'username'@'localhost' IDENTIFIED BY 'password';


Be aware, though, that the PRIVILEGES keyword is required for granting privileges like this when Strict SQL mode is turned on.

Deleting a User

Use the following syntax to delete a database user profile:

DROP USER IF EXISTS username;


Note that this command will not by default delete any tables created by the deleted user, and attempts to access such tables may result in errors.

Selecting a Database

Before you can create a table, you first have to tell the RDBMS the database in which you’d like to create it. In MySQL and MariaDB, do so with the following syntax:

USE database;


In PostgreSQL, you must use the following command to select your desired database:

\connect database


Creating a Table

The following command structure creates a new table with the name <span class="highlight">table</span>, and includes two columns, each with their own specific data type:

CREATE TABLE table ( column_1 column_1_data_type, column_2 column_2_data_taype );


Deleting a Table

To delete a table entirely, including all its data, run the following:

DROP TABLE IF EXISTS table


Inserting Data into a Table

Use the following syntax to populate a table with one row of data:

INSERT INTO table ( column_A, column_B, column_C ) VALUES ( 'data_A', 'data_B', 'data_C' );


You can also populate a table with multiple rows of data using a single command, like this:

INSERT INTO table ( column_A, column_B, column_C ) VALUES ( 'data_1A', 'data_1B', 'data_1C' ),  ( 'data_2A', 'data_2B', 'data_2C' ), ( 'data_3A', 'data_3B', 'data_3C' );


Deleting Data from a Table

To delete a row of data from a table, use the following command structure. Note that <span class="highlight">value</span> should be the value held in the specified <span class="highlight">column</span> in the row that you want to delete:

DELETE FROM table WHERE column='value';


Note: If you do not include a WHERE clause in a DELETE statement, as in the following example, it will delete all the data held in a table, but not the columns or the table itself:

DELETE FROM table;


Changing Data in a Table

Use the following syntax to update the data held in a given row. Note that the WHERE clause at the end of the command tells SQL which row to update. <span class="highlight">value</span> is the value held in <span class="highlight">column_A</span> that aligns with the row you want to change.

Note: If you fail to include a WHERE clause in an UPDATE statement, the command will replace the data held in every row of the table.

UPDATE table SET column_1 = value_1, column_2 = value_2 WHERE column_A=value;


Inserting a Column

The following command syntax will add a new column to a table:

ALTER TABLE table ADD COLUMN column data_type;


Deleting a Column

A command following this structure will delete a column from a table:

ALTER TABLE table DROP COLUMN column;


Performing Basic Queries

To view all the data from a single column in a table, use the following syntax:

SELECT column FROM table;


To query multiple columns from the same table, separate the column names with a comma:

SELECT column_1, column_2 FROM table;


You can also query every column in a table by replacing the names of the columns with an asterisk (*). In SQL, asterisks act as placeholders to represent “all”:

SELECT * FROM table;


Using WHERE Clauses

You can narrow down the results of a query by appending the SELECT statement with a WHERE clause, like this:

SELECT column FROM table WHERE conditions_that_apply;


For example, you can query all the data from a single row with a syntax like the following. Note that <span class="highlight">value</span> should be a value held in both the specified <span class="highlight">column</span> and the row you want to query:

SELECT * FROM table WHERE column = value;


Working with Comparison Operators

A comparison operator in a WHERE clause defines how the specified column should be compared against the value. Here are some common SQL comparison operators:

Operator What it does = tests for equality != tests for inequality < tests for less-than > tests for greater-than <= tests for less-than or equal-to >= tests for greater-than or equal-to BETWEEN tests whether a value lies within a given range IN tests whether a row’s value is contained in a set of specified values EXISTS tests whether rows exist, given the specified conditions LIKE tests whether a value matches a specified string IS NULL tests for NULL values IS NOT NULL tests for all values other than NULL Working with Wildcards

SQL allows the use of wildcard characters. These are useful if you’re trying to find a specific entry in a table, but aren’t sure of what that entry is exactly.

Asterisks (*) are placeholders that represent “all,” this will query every column in a table:

SELECT * FROM table;


Percentage signs (%) represent zero or more unknown characters.

SELECT * FROM table WHERE column LIKE val%;


Underscores (_) are used to represent a single unknown character:

SELECT * FROM table WHERE column LIKE v_lue;


Counting Entries in a Column

The COUNT function is used to find the number of entries in a given column. The following syntax will return the total number of values held in <span class="highlight">column</span>:

SELECT COUNT(column) FROM table;


You can narrow down the results of a COUNT function by appending a WHERE clause, like this:

SELECT COUNT(column) FROM table WHERE column=value;


Finding the Average Value in a Column

The AVG function is used to find the average (in this case, the mean) amongst values held in a specific column. Note that the AVG function will only work with columns holding numeric values; when used on a column holding string values, it may return an error or 0:

SELECT AVG(column) FROM table;


Finding the Sum of Values in a Column

The SUM function is used to find the sum total of all the numeric values held in a column:

SELECT SUM(column) FROM table;


As with the AVG function, if you run the SUM function on a column holding string values it may return an error or just 0, depending on your RDBMS.

Finding the Largest Value in a Column

To find the largest numeric value in a column or the last value alphabetically, use the MAX function:

SELECT MAX(column) FROM table;


Finding the Smallest Value in a Column

To find the smallest numeric value in a column or the first value alphabetically, use the MIN function:

SELECT MIN(column) FROM table;


Sorting Results with ORDER BY Clauses

An ORDER BY clause is used to sort query results. The following query syntax returns the values from <span class="highlight">column_1</span> and <span class="highlight">column_2</span> and sorts the results by the values held in <span class="highlight">column_1</span> in ascending order or, for string values, in alphabetical order:

SELECT column_1, column_2 FROM table ORDER BY column_1;


To perform the same action, but order the results in descending or reverse alphabetical order, append the query with DESC:

SELECT column_1, column_2 FROM table ORDER BY column_1 DESC;


Sorting Results with GROUP BY Clauses

The GROUP BY clause is similar to the ORDER BY clause, but it is used to sort the results of a query that includes an aggregate function such as COUNT, MAX, MIN, or SUM. On their own, the aggregate functions described in the previous section will only return a single value. However, you can view the results of an aggregate function performed on every matching value in a column by including a GROUP BY clause.

The following syntax will count the number of matching values in <span class="highlight">column_2</span> and group them in ascending or alphabetical order:

SELECT COUNT(column_1), column_2 FROM table GROUP BY column_2;


To perform the same action, but group the results in descending or reverse alphabetical order, append the query with DESC:

SELECT COUNT(column_1), column_2 FROM table GROUP BY column_2 DESC;


Querying Multiple Tables with JOIN Clauses

JOIN clauses are used to create result sets that combine rows from two or more tables. A JOIN clause will only work if the two tables each have a column with an identical name and data type, as in this example:

SELECT table_1.column_1, table_2.column_2 FROM table_1 JOIN table_2 ON table_1.common_column=table_2.common_column;


This is an example of an INNER JOIN clause. An INNER JOIN will return all the records that have matching values in both tables, but won’t show any records that don’t have matching values.

It’s possible to return all the records from one of two tables, including values that do not have a corresponding match in the other table, by using an outer JOIN clause. Outer JOIN clauses are written as either LEFT JOIN or RIGHT JOIN.

A LEFT JOIN clause returns all the records from the “left” table and only the matching records from the “right” table. In the context of outer JOIN clauses, the left table is the one referenced in the FROM clause, and the right table is any other table referenced after the JOIN statement. The following will show every record from <span class="highlight">table_1</span> and only the matching values from <span class="highlight">table_2</span>. Any values that do not have a match in <span class="highlight">table_2</span> will appear as NULL in the result set:

SELECT table_1.column_1, table_2.column_2 FROM table_1 LEFT JOIN table_2 ON table_1.common_column=table_2.common_column;


A RIGHT JOIN clause functions the same as a LEFT JOIN, but it prints the all the results from the right table, and only the matching values from the left:

SELECT table_1.column_1, table_2.column_2 FROM table_1 RIGHT JOIN table_2 ON table_1.common_column=table_2.common_column;


Combining Multiple SELECT Statements with UNION Clauses

A UNION operator is useful for combining the results of two (or more) SELECT statements into a single result set:

SELECT column_1 FROM table UNION SELECT column_2 FROM table;


Additionally, the UNION clause can combine two (or more) SELECT statements querying different tables into the same result set:

SELECT column FROM table_1 UNION SELECT column FROM table_2;


Conclusion

This guide covers some of the more common commands in SQL used to manage databases, users, and tables, and query the contents held in those tables. There are, however, many combinations of clauses and operators that all produce unique result sets. If you’re looking for a more comprehensive guide to working with SQL, we encourage you to check out Oracle’s Database SQL Reference.

Additionally, if there are common SQL commands you’d like to see in this guide, please ask or make suggestions in the comments below.

Learn More

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The Complete SQL Bootcamp

The Complete Oracle SQL Certification Course

SQL for Newbs: Data Analysis for Beginners

The Ultimate MySQL Bootcamp: Go from SQL Beginner to Expert

What are the differences between Standard SQL and Transact-SQL?

What are the differences between Standard SQL and Transact-SQL?

In this article, we'll explain syntax differences between standard SQL and the Transact-SQL language dedicated to interacting with the SQL

#1 Names of Database Objects

In relational database systems, we name tables, views, and columns, but sometimes we need to use the same name as a keyword or use special characters. In standard SQL, you can place this kind of name in quotation marks (""), but in T-SQL, you can also place it in brackets ([]). Look at these examples for the name of a table in T-SQL:

CREATE TABLE dbo.test.“first name” ( Id INT, Name VARCHAR(100));
CREATE TABLE dbo.test.[first name]  ( Id INT, Name VARCHAR(100));

Only the first delimiter (the quotation marks) for the special name is also part of the SQL standard.

What Is Different in a SELECT Statement?#2 Returning Values

The SQL standard does not have a syntax for a query returning values or values coming from expressions without referring to any columns of a table, but MS SQL Server does allow for this type of expression. How? You can use a SELECT statement alone with an expression or with other values not coming from columns of the table. In T-SQL, it looks like the example below:

SELECT 12/6 ;

In this expression, we don’t need a table to evaluate 12 divided by 6, therefore, the FROM statement and the name of the table can be omitted.

#3 Limiting Records in a Result Set

In the SQL standard, you can limit the number of records in the results by using the syntax illustrated below:

SELECT * FROM tab FETCH FIRST 10 ROWS ONLY

T-SQL implements this syntax in a different way. The example below shows the MS SQL Server syntax:

SELECT * FROM tab ORDER BY col1 DESC OFFSET 0 ROWS FETCH FIRST 10 ROWS ONLY;

As you notice, this uses an ORDER BY clause. Another way to select rows, but without ORDER BY, is by using the TOP clause in T-SQL:

SELECT TOP 10 * FROM tab;
#4 Automatically Generating Values

The SQL standard enables you to create columns with automatically generated values. The syntax to do this is shown below:

CREATE TABLE tab (id DECIMAL GENERATED ALWAYS AS IDENTITY);

In T-SQL we can also automatically generate values, but in this way:

CREATE TABLE tab (id INTEGER IDENTITY);
#5 Math Functions

Several common mathematical functions are part of the SQL standard. One of these math functions is CEIL(x), which we don’t find in T-SQL. Instead, T-SQL provides the following non-standard functions: SIGN(x), ROUND(x,[,d]) to round decimal value x to the number of decimal positions, TRUNC(x) for truncating to given number of decimal places, LOG(x) to return the natural logarithm for a value x, and RANDOM() to generate random numbers. The highest or lowest number in a list in the SQL standard is returned by MAX(list) and MIN(list) functions, but in Transact-SQL, you use the GREATEST(list) and LEAST(list) functions.

T-SQL function ROUND:

SELECT ROUND(col) FROM tab;

#6 Aggregate Functions

We find another syntax difference with the aggregate functions. The functions COUNT, SUM, and AVG all take an argument related to a count. T-SQL allows the use of DISTINCT before these argument values so that rows are counted only if the values are different from other rows. The SQL standard doesn't allow for the use of DISTINCT in these functions.

Standard SQL:
SELECT COUNT(col) FROM tab;

T-SQL:
SELECT COUNT(col) FROM tab;

SELECT COUNT(DISTINCT col) FROM tab;

But in T-SQL we don’t find a population covariance function: COVAR_POP(x,y), which is defined in the SQL standard.

#7 Retrieving Parts of Dates and Times

Most relational database systems deliver many functions to operate on dates and times.

In standard SQL, the EXTRACT(YEAR FROM x) function and similar functions to select parts of dates are different from the T-SQL functions like YEAR(x) or DATEPART(year, x).

There is also a difference in getting the current date and time. Standard SQL allows you to get the current date with the CURRENT_DATE function, but in MS SQL Server, there is not a similar function, so we have to use the GETDATE function as an argument in the CAST function to convert to a DATE data type.

#8 Operating on Strings

Using functions to operate on strings is also different between the SQL standard and T-SQL. The main difference is found in removing trailing and leading spaces from a string. In standard SQL, there is the TRIM function, but in T-SQL, there are several related functions: TRIM (removing trailing and leading spaces), LTRIM (removing leading spaces), and RTRIM (removing trailing spaces).

Another very-often-used string function is SUBSTRING.

The standard SQL syntax for the SUBSTRING function looks like:

SUBSTRING(str FROM start [FOR len])

but in T-SQL, the syntax of this function looks like:

SUBSTRING(str, start, length)

There are reasons sometimes to add values coming from other columns and/or additional strings. Standard SQL enables the following syntax to do this:

As you can see, this syntax makes use of the || operator to add one string to another.

But the equivalent operator in T-SQL is the plus sign character. Look at this example:

SELECT col1 + col2  FROM tab;

In SQL Server, we also have the possibility to use the CONCAT function concatenates a list of strings:

SELECT CONCAT(col1, str1, col2, ...)  FROM tab;

We can also repeat one character several times. Standard SQL defines the function REPEAT(str, n) to do this. Transact-SQL provides the REPLICATE function. For example:

SELECT  REPLICATE(str, x);

where x indicates how many times to repeat the string or character.

#9 Inequality Operator

During filtering records in a SELECT statement, sometimes we have to use an inequality operator. Standard SQL defines <> as this operator, while T-SQL allows for both the standard operator and the != operator:

SELECT col3 FROM tab WHERE col1 != col2;
#10 ISNULL Function

In T-SQL, we have the ability to replace NULL values coming from a column using the ISNULL function. This is a function that is specific to T-SQL and is not in the SQL standard.

SELECT ISNULL(col1) FROM tab;
Which Parts of DML Syntax Are Different?

In T-SQL, the basic syntax of DELETE, UPDATE, and INSERT queries is the same as the SQL standard, but differences appear in more advanced queries. Let’s look at them.

#11 OUTPUT Keyword

The OUTPUT keyword occurs in DELETE, UPDATE, and INSERT statements. It is not defined in standard SQL.

Using T-SQL, we can see extra information returned by a query. It returns both old and new values in UPDATE or the values added using INSERT or deleted using DELETE. To see this information, we have to use prefixes in INSERT, UPDATE, and DELETE.

UPDATE tab SET col='new value'
OUTPUT Deleted.col, Inserted.col;

We see the result of changing records with the previous and new values in an updated column. The SQL standard does not support this feature.

#12 Syntax for INSERT INTO ... SELECT

Another structure of an INSERT query is INSERT INTO … SELECT. T-SQL allows you to insert data from another table into a destination table. Look at this query:

INSERT INTO tab SELECT col1,col2,... FROM tab_source;

It is not a standard feature but a feature characteristic of SQL Server.

#13 FROM Clause in DELETE and UPDATE

SQL Server provides extended syntax of the UPDATE and DELETE with FROM clauses. You can use DELETE with FROM to use the rows from one table to remove corresponding rows in another table by referring to a primary key and a foreign key. Similarly, you can use UPDATE with FROM update rows from one table by referring to the rows of another table using common values (primary key in one table and foreign key in second, e.g. the same city name). Here is an example:

DELETE FROM Book
FROM Author
WHERE Author.Id=Book.AuthorId AND Author.Name IS NULL;

UPDATE Book
SET Book.Price=Book.Price*0.2
FROM Author
WHERE Book.AuthorId=Author.Id AND Author.Id=12;

The SQL standard doesn’t provide this syntax.

#14 INSERT, UPDATE, and DELETE With JOIN

You can also use INSERT, UPDATE, and DELETE using JOIN to connect to another table. An example of this is:

DELETE ItemOrder FROM ItemOrder
JOIN Item ON ItemOrder.ItemId=Item.Id
WHERE YEAR(Item.DeliveredDate) <= 2017;

This feature is not in the SQL standard.

Summary

This article does not cover all the issues about syntax differences between the SQL standard and T-SQL using the MS SQL Server system. However, this guide helps point out some basic features characteristic only of Transact-SQL and what SQL standard syntax isn’t implemented by MS SQL Server.

Thanks for reading. If you liked this post, share it with all of your programming buddies!

Originally published on https://dzone.com