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FROM SQL operator
FROM SQL statement is used to enumerate tables and any associations required for SQL statement.
Syntax of FROM statement in SQL
FROM tab1
[ { INNER JOIN
| LEFT [OUTER] JOIN
| RIGHT [OUTER] JOIN
| FULL [OUTER] JOIN } tab2
ON tab1.col1 = tab2.col1 ]
where:
- tab1 and tab2 – These are tables used in an SQL statement. Two tables are combined based on tab1.col1 = tab2.col1.
Note – In FROM SQL operator at least one table must be specified.
If two or more tables are specified in FROM operator, these tables are usually united using INNER or OUTER unions.
FROM operator with one table
We will begin by considering how to use FROM in an SQL query where only one table is specified.
In this example, we have a table with the following data:
| suppl_id | suppl_name | city_id | state_id |
|---|---|---|---|
| 100 | Yandex | Moscow | Russian |
| 200 | Lansing | Michigan | |
| 300 | Oracle | Redwood City | California |
| 400 | Bing | Redmond | Washington |
| 500 | Yahoo | Sunnyvale | Washington |
| 600 | DuckDuckGo | Paoli | Pennsylvania |
| 700 | Qwant | Paris | France |
| 800 | Menlo Park | California | |
| 900 | Electronic Arts | San Francisco | California |
Enter the following SQL statement:
SELECT * FROM supplsWHERE suppl_id < 400ORDER BY city_id DESC;Three records will be selected. Here are the results that you should get:
| suppl_id | suppl_name | city_id | state_id |
|---|---|---|---|
| 100 | Yandex | Moscow | Russian |
| 200 | Lansing | Michigan | |
| 300 | Oracle | Redwood City | California |
In this example, we used the FROM operator to display the table. There are no executed connections in this query since we have listed only one table in FROM.
Two tables in the FROM operator (INNER JOIN)
Let’s see how to use the FROM sentence to INNER JOIN the two tables together.
In this example, we have a product table with the following data:
| prod_id | prod_name | cat_id |
|---|---|---|
| 1 | Pear | 50 |
| 2 | Banana | 50 |
| 3 | Orange | 50 |
| 4 | Apple | 50 |
| 5 | Bread | 75 |
| 6 | Sliced Ham | 25 |
| 7 | Kleenex | NULL |
And the cats table with the following data:
| cat_id | cat_name |
|---|---|
| 25 | Deli |
| 50 | Produce |
| 75 | Bakery |
| 100 | General Merchandise |
| 125 | Technology |
Perform the following SQL statement:
SELECT prods.prod_name, cats.cat_nameFROMprodsINNER JOIN catsON products.cat_id = cats.cat_idWHERE prod_name <> 'Pear';| prod_name | cat_name |
|---|---|
| Banana | Produce |
| Orange | Produce |
| Apple | Produce |
| Bread | Bakery |
| Sliced Ham | Deli |
In this example of the FROM operator, the two tables – prods and cats – are combined. In this case, we use FROM to specify INNER JOIN between the product and category tables based on the category_id column in both tables.
Two tables in the FROM operator (OUTER JOIN)
Consider how to use FROM when we connect two tables together using OUTER JOIN. In this case we will look at the LEFT OUTER JOIN.
Let us use the same tables and categories as in the INNER JOIN example above, but this time we will combine the tables using the LEFT OUTER JOIN. Enter the following SQL statement:
SELECT prods.prod_name, cat.cat_name FROMprods LEFT OUTER JOIN cats ON prods.cat_id = cats.cat_idWHERE prod_name <> 'Pear';Six records will be selected. Here are the results that you should get:
| prod_name | cat_name |
|---|---|
| Banana | Produce |
| Orange | Produce |
| Apple | Produce |
| Bread | Bakery |
| Sliced Ham | Deli |
| Kleenex | NULL |
In this FROM example, LEFT OUTER JOIN is used with the cat_id based products and categories tables in both tables.
Now our result set will have the last entry with prod_name ‘Kleenex’ with the value NULL for cat_name. This entry did not appear in our results when we executed INNER JOIN.
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