Preamble
PostgreSQL SELECT statement is used to extract records from one or more tables into PostgreSQL.
In its simplest form, the syntax for the SELECT statement in PostgreSQL
SELECT_id
FROM tabs
[WHERE conds];
The full syntax for the SELECT PostgreSQL statement
SELECT [ ALL | DISTINCT | DISTINCT ON (distinct_expressions_id) ]
expressions_id
FROM tabs
[WHERE conds]
[GROUP BY_id]
[HAVING cond]
[ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS FIRST | NULLS LAST ]]
[LIMIT [ number_rows | ALL]
[OFFSET offset_value [ ROW | ROWS ]]
[FETCH { FIRST | NEXT } [ fetch_rows ] { ROW | ROWS } ONLY]
[FOR { UPDATE | SHARE } OF table [ NOWAIT ]];
Parameters and arguments of the statement
- ALL – Optional. Returns all matching strings.
- DISTINCT – Optional. Removes duplicates from the result set. Learn more about the DISTINCT operator.
- DISTINCT ON – Optional. Removes duplicates based on different_expressions. Learn more about the DISTINCT ON operator.
- expressions_id – The table or calculations you want to get. Use * if you want to select all columns.
- tabs – The tables from which you want to get the records. At least one table must be specified in the FROM statement.
- WHERE conds – Optional. The conditions that must be met for the records to be selected.
- GROUP BY_id – Optional. It collects data from several records and groups the results into one or more columns.
- HAVING cond – Optional. It is used in combination with GROUP BY to limit groups of returned rows to only those whose TRUE condition.
- ORDER BY expression_id – Optional. It is used to sort the entries in your resulting set.
- LIMIT – Optional. If LIMIT is specified, it controls the maximum number of records to extract. The maximum number of records specified by the number_rows will be returned in the resulting set. The first line returned by LIMIT will be defined as offset_value.
- FETCH – Optional. If FETCH is specified, it controls the maximum number of records to extract. At most, the maximum number of records specified by fetch_rows will be returned in the resulting set. The first line returned by FETCH will be defined as offset_value.
- FOR UPDATE – Optional. Records affected by the query are blocked from being written until the transaction is completed.
- FOR SHARE – Optional. Records affected by a query may be used by other transactions, but cannot be updated or deleted by those other transactions.
Example: select all fields from one table
Let’s see how to use PostgreSQL query SELECT to select all fields in a table.
SELECT *
FROM categories
WHERE category_id >= 2500
ORDER BY category_id ASC;
In this example of the SELECT PostgreSQL statement, we used * to specify that we want to select all fields from the category table where category_id is greater than or equal to 2500. The resulting set is sorted by category_id in ascending order.
Example: select individual fields from one table
You can also use PostgreSQL statement SELECT to select individual fields from a table, unlike all fields from a table.
For example:
SELECT category_id, category_name, comments
FROM categories
WHERE category_name = 'Hardware'
ORDER BY category_name ASC, comments DESC;
This PostgreSQL example SELECT will only return the category_id, category_name, and comments fields from the category table, where category_name is ‘Hardware’. The results are sorted by category_name in ascending order and then by comments in descending order.
Example: selecting fields from several tables
You can also use the SELECT operator of PostgreSQL to extract fields from multiple tables.
SELECT products.product_name, categories.category_name
FROM categories
INNER JOIN products
ON categories.category_id = products.category_id
ORDER BY product_name;
This PostgreSQL SELECT example connects two tables to get the resulting set, which displays the product_name and category_name fields where the category_id value matches in both the categories and product tables. The results are sorted by product_name in ascending order.
PostgreSQL: Select From | Course
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