Sequences and auto-increment in Snowflake - Step-by-Step Execution

Process Flow - Sequences and auto-increment

Create Sequence

↓

Sequence Initialized

↓

Request Next Value

↓

Sequence Returns Current Value

↓

Sequence Increments

↓

Use Value in Table Insert

↓

Repeat Request for Next Value

This flow shows how a sequence is created, then each time a next value is requested, the sequence returns the current number and increments for the next call.

Execution Sample

Snowflake

CREATE SEQUENCE my_seq START WITH 1 INCREMENT BY 1;
SELECT my_seq.nextval;
INSERT INTO my_table (id, name) VALUES (my_seq.nextval, 'Alice');
SELECT my_seq.nextval;

This code creates a sequence starting at 1, gets the next value twice, and inserts a row using the sequence value.

Process Table

Step	Action	Sequence Current Value	Returned Value	Notes
1	Create sequence my_seq START WITH 1 INCREMENT BY 1	0 (initialized)	N/A	Sequence created but no value generated yet
2	SELECT my_seq.nextval	1	1	First call returns 1 and increments sequence to 1
3	INSERT INTO my_table (id, name) VALUES (my_seq.nextval, 'Alice')	2	2	Nextval returns 2 for insert, sequence increments to 2
4	SELECT my_seq.nextval	3	3	Nextval returns 3, sequence increments to 3
5	No more calls	3	N/A	Sequence holds last value 3, no further increments

💡 Execution stops after no more nextval calls; sequence holds last incremented value.

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
my_seq.current_value	0	1	2	3	3
Returned nextval	N/A	1	2	3	N/A

Key Moments - 3 Insights

Why does the sequence start at 0 but the first nextval returns 1?

The sequence is initialized at 0 internally but the first nextval call increments it to the start value 1 before returning, as shown in execution_table step 2.

Does calling nextval change the sequence value even if not used in insert?

Yes, each nextval call increments the sequence regardless of usage, as seen in steps 2, 3, and 4 where current_value increments each time.

What happens if you call nextval multiple times without inserting?

Each call increments and returns the next number, so the sequence value advances even without inserts, shown by the increments in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the sequence current value after step 3?

A 2

B 1

C 3

D 0

💡 Hint

Check the 'Sequence Current Value' column for step 3 in execution_table.

At which step does the sequence return the value 3?

A Step 2

B Step 3

C Step 4

D Step 1

💡 Hint

Look at the 'Returned Value' column in execution_table to find when 3 is returned.

If you call nextval twice more after step 4, what will be the returned values?

A 5 and 6

B 4 and 5

C 3 and 4

D 2 and 3

💡 Hint

Sequence increments by 1 each nextval call; after 3, next values are 4 then 5.

Concept Snapshot

CREATE SEQUENCE name START WITH x INCREMENT BY y;
Use name.nextval to get next number.
Each nextval call returns current value and increments.
Useful for unique IDs in tables.
Sequence state persists until dropped.

Full Transcript

This visual execution shows how Snowflake sequences work. First, a sequence is created with a start and increment value. Initially, the sequence holds no value. When nextval is called, it returns the current sequence number and increments it for the next call. This number can be used in inserts to provide unique IDs. Each call to nextval advances the sequence, even if the value is not used. The sequence keeps track of its current value until the sequence is dropped or reset.

Practice

1. What is the main purpose of a sequence in Snowflake?

easy

A. To create backup copies of tables

B. To store large text data efficiently

C. To automatically generate unique numeric values

D. To manage user permissions

Solution

1. Step 1: Understand what a sequence does

   A sequence generates numbers automatically, usually for unique IDs.

2. Step 2: Identify the correct purpose

   Among the options, only automatic unique number generation matches the sequence's role.

3. Final Answer:

   To automatically generate unique numeric values -> Option C

4. Quick Check:

   Sequence = unique number generator [OK]

Hint: Sequences generate unique numbers automatically [OK]

Common Mistakes:

• Confusing sequences with tables
• Thinking sequences store text data
• Assuming sequences manage permissions

2. Which of the following is the correct syntax to get the next value from a sequence named order_seq?

easy

A. SELECT order_seq.NEXTVAL();

B. SELECT NEXTVAL('order_seq');

C. SELECT NEXTVAL(order_seq);

D. SELECT order_seq.nextval;

Solution

1. Step 1: Recall Snowflake sequence syntax

   Snowflake uses sequence_name.NEXTVAL() dot notation to get the next value.

2. Step 2: Match syntax to options

   Only SELECT order_seq.NEXTVAL(); uses the correct syntax.

3. Final Answer:

   SELECT order_seq.NEXTVAL(); -> Option A

4. Quick Check:

   Use seq_name.NEXTVAL() syntax [OK]

Hint: Use seq_name.NEXTVAL() [OK]

Common Mistakes:

• Omitting quotes around sequence name
• Passing non-string to NEXTVAL()
• Missing parentheses on NEXTVAL call

3. Given the sequence user_seq starting at 1000 and incrementing by 5, what will be the output of these two queries executed in order?

SELECT NEXTVAL('user_seq');
SELECT NEXTVAL('user_seq');

medium

A. 1005 and 1010

B. 1000 and 1001

C. 1000 and 1005

D. 1 and 2

Solution

1. Step 1: Understand sequence start and increment

   The sequence starts at 1000 and adds 5 each time NEXTVAL is called.

2. Step 2: Calculate the two NEXTVAL calls

   First call returns 1005, second call returns 1010.

3. Final Answer:

   1005 and 1010 -> Option A

4. Quick Check:

   Start=1000, increment=5, next two values = 1005, 1010 [OK]

Hint: First value = start + increment, next adds increment again [OK]

Common Mistakes:

• Assuming first NEXTVAL returns start value
• Confusing start value with first increment
• Thinking sequence resets after one call

4. You created a sequence with:
CREATE SEQUENCE invoice_seq START = 1 INCREMENT = 1;
But when you run SELECT NEXTVAL('invoice_seq'); you get an error. What is the most likely cause?

medium

A. Sequence name must be uppercase

B. Sequence was not committed or created properly

C. NEXTVAL function does not exist in Snowflake

D. You must specify schema name in NEXTVAL

Solution

1. Step 1: Check sequence creation and commit

   Sequences must be created successfully and committed before use.

2. Step 2: Identify common error cause

   If NEXTVAL errors, often the sequence does not exist or creation failed.

3. Final Answer:

   Sequence was not committed or created properly -> Option B

4. Quick Check:

   Sequence must exist before NEXTVAL call [OK]

Hint: Ensure sequence creation succeeded before NEXTVAL [OK]

Common Mistakes:

• Assuming case sensitivity causes error
• Thinking NEXTVAL is unsupported
• Forgetting to commit or check creation

5. You want to create a table orders with an auto-incrementing order_id using a sequence order_seq. Which approach correctly assigns order_id during insert?

hard

A. Create sequence and use INSERT INTO orders VALUES (order_seq + 1, ...);

B. Create sequence and use INSERT INTO orders VALUES (NEXTVAL('order_seq'), ...);

C. Create sequence and manually increment order_id in application code

D. Create sequence and set order_id default to order_seq.NEXTVAL()

Solution

1. Step 1: Understand how to use sequences in inserts

   Sequences provide unique numbers via NEXTVAL function during insert statements.

2. Step 2: Evaluate options for assigning order_id

   Only B correctly sets the order_id column default to order_seq.NEXTVAL() for automatic assignment on insert (when column omitted).

3. Final Answer:

   Create sequence and set order_id default to order_seq.NEXTVAL() -> Option D

4. Quick Check:

   Column DEFAULT seq.NEXTVAL() [OK]

Hint: Set column DEFAULT seq_name.NEXTVAL() [OK]

Common Mistakes:

• Trying to use sequence name directly without NEXTVAL
• Manually incrementing IDs in app code
• Using invalid syntax like order_seq + 1