MERGE 7 SQL - Language Statements MERGE insert, update, or delete rows of a table based upon source data MERGE INTO target_table_name [ [ AS ] target_alias ] USING data_source ON join_condition when_clause [...] where data_source is { source_table_name | ( source_query ) } [ [ AS ] source_alias ] and when_clause is { WHEN MATCHED [ AND condition ] THEN { merge_update | merge_delete } | WHEN NOT MATCHED [ AND condition ] THEN { merge_insert | DO NOTHING } } and merge_insert is INSERT [( column_name [, ...] )] [ OVERRIDING { SYSTEM | USER } VALUE ] { VALUES ( { expression | DEFAULT } [, ...] ) | DEFAULT VALUES } and merge_update is UPDATE SET { column_name = { expression | DEFAULT } | ( column_name [, ...] ) = ( { expression | DEFAULT } [, ...] ) } [, ...] and merge_delete is DELETE MERGE performs actions that modify rows in the target_table_name, using the data_source. MERGE provides a single SQL statement that can conditionally INSERT, UPDATE or DELETE rows, a task that would otherwise require multiple procedural language statements. First, the MERGE command performs a join from data_source to target_table_name producing zero or more candidate change rows. For each candidate change row the status of MATCHED or NOT MATCHED is set just once, after which WHEN clauses are evaluated in the order specified. If one of them is activated, the specified action occurs. No more than one WHEN clause can be activated for any candidate change row. MERGE actions have the same effect as regular UPDATE, INSERT, or DELETE commands of the same names. The syntax of those commands is different, notably that there is no WHERE clause and no tablename is specified. All actions refer to the target_table_name, though modifications to other tables may be made using triggers. When DO NOTHING action is specified, the source row is skipped. Since actions are evaluated in the given order, DO NOTHING can be handy to skip non-interesting source rows before more fine-grained handling. There is no MERGE privilege. You must have the UPDATE privilege on the column(s) of the target_table_name referred to in the SET clause if you specify an update action, the INSERT privilege on the target_table_name if you specify an insert action and/or the DELETE privilege on the target_table_name if you specify a delete action on the target_table_name. Privileges are tested once at statement start and are checked whether or not particular WHEN clauses are activated during the subsequent execution. You will require the SELECT privilege on the data_source and any column(s) of the target_table_name referred to in a condition. MERGE is not supported if the target_table_name has RULES defined on it. See for more information about RULES. target_table_name The name (optionally schema-qualified) of the target table to merge into. target_alias A substitute name for the target table. When an alias is provided, it completely hides the actual name of the table. For example, given MERGE foo AS f, the remainder of the MERGE statement must refer to this table as f not foo. source_table_name The name (optionally schema-qualified) of the source table, view or transition table. source_query A query (SELECT statement or VALUES statement) that supplies the rows to be merged into the target_table_name. Refer to the statement or statement for a description of the syntax. source_alias A substitute name for the data source. When an alias is provided, it completely hides whether table or query was specified. join_condition join_condition is an expression resulting in a value of type boolean (similar to a WHERE clause) that specifies which rows in the data_source match rows in the target_table_name. Only columns from target_table_name that attempt to match data_source rows should appear in join_condition. join_condition subexpressions that only reference target_table_name columns can only affect which action is taken, often in surprising ways. when_clause At least one WHEN clause is required. If the WHEN clause specifies WHEN MATCHED and the candidate change row matches a row in the target_table_name the WHEN clause is activated if the condition is absent or is present and evaluates to true. If the WHEN clause specifies WHEN NOT MATCHED and the candidate change row does not match a row in the target_table_name the WHEN clause is activated if the condition is absent or is present and evaluates to true. condition An expression that returns a value of type boolean. If this expression returns true then the WHEN clause will be activated and the corresponding action will occur for that row. The expression may not contain functions that possibly performs writes to the database. A condition on a WHEN MATCHED clause can refer to columns in both the source and the target relation. A condition on a WHEN NOT MATCHED clause can only refer to columns from the source relation, since by definition there is no matching target row. Only the system attributes from the target table are accessible. merge_insert The specification of an INSERT action that inserts one row into the target table. The target column names can be listed in any order. If no list of column names is given at all, the default is all the columns of the table in their declared order. Each column not present in the explicit or implicit column list will be filled with a default value, either its declared default value or null if there is none. If the expression for any column is not of the correct data type, automatic type conversion will be attempted. If target_table_name is a partitioned table, each row is routed to the appropriate partition and inserted into it. If target_table_name is a partition, an error will occur if one of the input rows violates the partition constraint. Column names may not be specified more than once. INSERT actions cannot contain sub-selects. The VALUES clause can only refer to columns from the source relation, since by definition there is no matching target row. merge_update The specification of an UPDATE action that updates the current row of the target_table_name. Column names may not be specified more than once. Do not include the table name, as you would normally do with an command. For example, UPDATE tab SET col = 1 is invalid. Also, do not include a WHERE clause, since only the current row can be updated. For example, UPDATE SET col = 1 WHERE key = 57 is invalid. merge_delete Specifies a DELETE action that deletes the current row of the target_table_name. Do not include the tablename or any other clauses, as you would normally do with an command. column_name The name of a column in the target_table_name. The column name can be qualified with a subfield name or array subscript, if needed. (Inserting into only some fields of a composite column leaves the other fields null.) When referencing a column, do not include the table's name in the specification of a target column. OVERRIDING SYSTEM VALUE Without this clause, it is an error to specify an explicit value (other than DEFAULT) for an identity column defined as GENERATED ALWAYS. This clause overrides that restriction. OVERRIDING USER VALUE If this clause is specified, then any values supplied for identity columns defined as GENERATED BY DEFAULT are ignored and the default sequence-generated values are applied. DEFAULT VALUES All columns will be filled with their default values. (An OVERRIDING clause is not permitted in this form.) expression An expression to assign to the column. The expression can use the old values of this and other columns in the table. DEFAULT Set the column to its default value (which will be NULL if no specific default expression has been assigned to it). On successful completion, a MERGE command returns a command tag of the form MERGE total-count The total-count is the total number of rows changed (whether inserted, updated, or deleted). If total-count is 0, no rows were changed in any way. The following steps take place during the execution of MERGE. Perform any BEFORE STATEMENT triggers for all actions specified, whether or not their WHEN clauses are activated during execution. Perform a join from source to target table. The resulting query will be optimized normally and will produce a set of candidate change row. For each candidate change row Evaluate whether each row is MATCHED or NOT MATCHED. Test each WHEN condition in the order specified until one activates. When activated, perform the following actions Perform any BEFORE ROW triggers that fire for the action's event type. Apply the action specified, invoking any check constraints on the target table. However, it will not invoke rules. Perform any AFTER ROW triggers that fire for the action's event type. Perform any AFTER STATEMENT triggers for actions specified, whether or not they actually occur. This is similar to the behavior of an UPDATE statement that modifies no rows. In summary, statement triggers for an event type (say, INSERT) will be fired whenever we specify an action of that kind. Row-level triggers will fire only for the one event type activated. So a MERGE might fire statement triggers for both UPDATE and INSERT, even though only UPDATE row triggers were fired. You should ensure that the join produces at most one candidate change row for each target row. In other words, a target row shouldn't join to more than one data source row. If it does, then only one of the candidate change rows will be used to modify the target row, later attempts to modify will cause an error. This can also occur if row triggers make changes to the target table which are then subsequently modified by MERGE. If the repeated action is an INSERT this will cause a uniqueness violation while a repeated UPDATE or DELETE will cause a cardinality violation; the latter behavior is required by the SQL Standard. This differs from historical PostgreSQL behavior of joins in UPDATE and DELETE statements where second and subsequent attempts to modify are simply ignored. If a WHEN clause omits an AND clause it becomes the final reachable clause of that kind (MATCHED or NOT MATCHED). If a later WHEN clause of that kind is specified it would be provably unreachable and an error is raised. If a final reachable clause is omitted it is possible that no action will be taken for a candidate change row. The order in which rows are generated from the data source is indeterminate by default. A source_query can be used to specify a consistent ordering, if required, which might be needed to avoid deadlocks between concurrent transactions. There is no RETURNING clause with MERGE. Actions of INSERT, UPDATE and DELETE cannot contain RETURNING or WITH clauses. You may also wish to consider using INSERT ... ON CONFLICT as an alternative statement which offers the ability to run an UPDATE if a concurrent INSERT occurs. There are a variety of differences and restrictions between the two statement types and they are not interchangeable. Perform maintenance on CustomerAccounts based upon new Transactions. MERGE CustomerAccount CA USING RecentTransactions T ON T.CustomerId = CA.CustomerId WHEN MATCHED THEN UPDATE SET Balance = Balance + TransactionValue WHEN NOT MATCHED THEN INSERT (CustomerId, Balance) VALUES (T.CustomerId, T.TransactionValue); notice that this would be exactly equivalent to the following statement because the MATCHED result does not change during execution MERGE CustomerAccount CA USING (Select CustomerId, TransactionValue From RecentTransactions) AS T ON CA.CustomerId = T.CustomerId WHEN NOT MATCHED THEN INSERT (CustomerId, Balance) VALUES (T.CustomerId, T.TransactionValue) WHEN MATCHED THEN UPDATE SET Balance = Balance + TransactionValue; 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. Don't allow entries that have zero stock. MERGE INTO wines w USING wine_stock_changes s ON s.winename = w.winename WHEN NOT MATCHED AND s.stock_delta > 0 THEN INSERT VALUES(s.winename, s.stock_delta) WHEN MATCHED AND w.stock + s.stock_delta > 0 THEN UPDATE SET stock = w.stock + s.stock_delta; WHEN MATCHED THEN DELETE; The wine_stock_changes table might be, for example, a temporary table recently loaded into the database. This command conforms to the SQL standard. The DO NOTHING action is an extension to the SQL standard.