Postgres update select examples

Skip to content. What is SQL? SQL is a powerful language where we can perform a wide range of operations: execute queries fetch data insert, update, and delete records in a database DML operations create new objects in a database DDL operations set permissions on tables, procedures, functions, and views and much, much more Comparison Operators in PostgreSQL queries In PostgreSQL, with the help of comparison operators we can find results where the value in a column is not equal to the specified condition or value.

Hierarchical queries in PostgreSQL Hierarchical queries are ones where the results have a structured or parent-child relationship and are displayed in a tree structure. Length function in PostgreSQL The length function returns the number of characters or number of bytes in a specified string variable. Exporting query result to a text file in PostgreSQL With the help of the COPY command , we can export data from a table to an outside text file as well as import data from a text file into a table.

PostgreSQL recursive query Recursive queries are used to deal with hierarchical queries or tree-structured data. Enum query in PostgreSQL Enumerated enum types are data types that comprise a static, ordered set of values. Pivot query in PostgreSQL A pivot table is a useful way to analyze large quantities of data by organizing it into a more manageable format. Prepared statements can accept parameters. Running a query in a shell script in PostgreSQL We can use the bash shell script to execute psql commands.

Get path of bin directory. Query optimization in PostgreSQL These are a few of the tools available for improving PostgreSQL query performance: Unfortunately there is no single fixed formula for the optimization of PostgreSQL queries, as query performance can be affected by many things for example, performance could be affected by a hardware issue. Popular Links. Featured Links. PostgreSQL vs.

The count is the number of rows updated. If count is 0, no rows matched the condition this is not considered an error. When using FROM you should ensure that the join produces at most one output row for each row to be modified. In other words, a target row shouldn't join to more than one row from the other table s.

If it does, then only one of the join rows will be used to update the target row, but which one will be used is not readily predictable. Because of this indeterminacy, referencing other tables only within sub-selects is safer, though often harder to read and slower than using a join. Change the word Drama to Dramatic in the column kind of the table films :. Adjust temperature entries and reset precipitation to its default value in one row of the table weather :. Increment the sales count of the salesperson who manages the account for Acme Corporation, using the FROM clause syntax:.

Attempt to insert a new stock item along with the quantity of stock. If the item already exists, instead update the stock count of the existing item. To do this without failing the entire transaction, use savepoints:. According to the standard, the column-list syntax should allow a list of columns to be assigned from a single row-valued expression, such as a sub-select:.

We can use the select This query will error out if the rows are not available for selection. Select for update can be a rigid lock on your table. Concurrent processes can be blocked and starved out. Waiting is the slowest form of concurrent processing. If only one CPU can be active at a time, it is pointless to scale your servers. For this purpose, in PostgreSQL there is a mechanism for selecting only rows that are not locked.

One thing that we need to keep in mind while working with select for update statements is its effect on foreign keys. When selecting data from the purchases table with select for update , users will be locked as well. This is necessary because otherwise there is a chance of breaking the foreign-key constraint.

In bigger systems, a select for share can have huge negative consequences if it locks a widely used table. Keep in mind that other processes will only need to wait if they want to update the referenced field. If the other process wants to update some unrelated data, no blocking will occur. A weaker form of select for update is the select for share query. It is an ideal for ensuring referential integrity when creating child records for a parent. Suppose that we want to create a new purchase for a user.